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USMLE World Missed Questions > Microbiology, Immunology, Misc. > Flashcards

Microbiology, Immunology, Misc. Flashcards

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USMLE® Step 1 flashcards
1
Q

What is the mechanism of action of diptheria toxin?

A

AB-exotoxin that ribosylates intracellular proteins. Ribosylation inhibits EF-a and thus protein synthesis (causing death).

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2
Q

ST elevation in leads V1- V3 indicates occlusion of what artery?

A

LAD.

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3
Q

What condition is associated with immune complex vasculitis of IgA and C3 deposition?

A

Henoch-Scholein Purpura.

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4
Q

What are some serious complications of Henoch-Scholein Purpura?

A

Glomerulonephritis and end stage renal disease.

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5
Q

What is the function of TNF-a?

A

Cytokine that induces the systemic inflammatory response; in high concentrations it causes septic shock and cachexia.

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6
Q

Name three cytokines responsible for the systemic inflammatory response.

A

TNF-a, IL-1, IL-6.

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7
Q

Which cytokine limits the production of pro-inflammatory cytokines?

A

IL-10.

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8
Q

How can HAV contamination be best contained?

A

Virus is inactivated with water, chlorination, bleach (1:100 dilution) formalin, UV irradiation, or boiling to 85 degC for one minute

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9
Q

What is the most likely outcome for a patient acutely infected with HCV?

A

Stable chronic hepatitis- most pts remain HCV RNA positive and have persistent elevation of LFTs. A smaller subset progresses to cirrhosis.

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10
Q

How does angioedema most commonly present?

A

As swelling of the lips, tongue, or eyelids. Pts may or may not have laryngeal edema and difficulty breathing.

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11
Q

What is a common cause of angioedema?

A

ACE inhibitors. They cause increased bradykinin levels and lead to kinin accumulation.

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12
Q

What is seen on wet mount microscopy of discharge in a patient with bacterial vaginosis?

A

Clue cells- vaginal squamous epithelial cells covered with multiple small adherent bacteria. (Grey white vaginal discharge)

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13
Q

What type of bacteria is gardnerella vaginalis and what condition does it cause?

A

BV; it is a gram variable coccobacilli.

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14
Q

What is the presentation of trichomoniasis?

A

Yellow-green foamy and foul smelling discharge.

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15
Q

What is seem on wet mount of Trichomonas vaginalis?

A

Motile flagellated trophozoites.

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16
Q

What type of bug is trichomonas vaginalis?

A

Flagellated trophozoite.

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17
Q

What are the predominant cells found in granulomas?

A

Epitheloid macrophages and multinucleated giant cells. They often form after tissue macrophages encounter pathogens or substances that cant be easily digested or removed.

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18
Q

How do M. tuberculosis bacteria survive?

A

They are engulfed by macrophages and evade intracellular killing to survive and reproduce within phagolysosomes; they are able to present mycobaterial antigens to naiive helper T cells.

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19
Q

What is the function of IL-12 secretion?

A

It induces T cells to differentiated into Th1 T cells which produce interferon-y, activating macrophages and enabling them to kill ingested mycobacteria.

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20
Q

What response occurs when macrophages produce TNa?

A

Recruitment of additional macropages and monocytes to the area.

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21
Q

Which three factors are critical for formation and maintence of granulomas?

A

Interferon-y, IL-12, TNF-a

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22
Q

What is the difference between herpesviruses and other enveloped nucleocapsid viruses in terms of their lipid bilayer?

A

Herpes viruses bud through and acquire their envelope from the host cell nuclear membrane; all other viruses acquire their envelope from budding through the plasma membrane of the host cell.

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23
Q

Name three diseases caused by exotoxin release by S. aureus.

A

TSS, Staph scalded syndrome, Gastroenteritis

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24
Q

Describe the sequela of sx of staphlococcal food poisioning.

A

Pt. eats a mayonnaise containing product inoculated by staph from the food handler. At room temp, S. aureus is incubated, producing a pre-formed heat stable exotoxin that causes rapid onset nausea, vomiting, abdominal cramping.

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25
Q

Describe the sequlea of sx of bacillus cereus food poisoning.

A

Contaminates food with a preformed heat-stable exotoxin (usually starchy foods- reheated fried rice esp.)

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26
Q

What condition may lower the minimum infective dose of V. Cholerae by many orders of magnitude?

A

Increased gastric pH which can be caused by achlorhydria, food ingestion, antacid ingestion.

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27
Q

What is the most common cause of infections associated with foreign bodies?

A

S. epidermidis.

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28
Q

What is the primary virulence factor of staph epidermidis?

A

Synthesis of an extracellular polysaccharide matrix (biofilm) that prevents antibiotic penetration and interferes with host defenses.

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29
Q

What bug is the primary cause of epiglotitis?

A

H. influenza type B.

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30
Q

What side effects are associated with Amitryptiline?

A

Anticholinergic due to block of muscarinic receptors; may mimic atropine toxicity.

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31
Q

What is the main adverse effect of prazosin?

A

hypotension (esp. postural)

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32
Q

How is strongyloidiasis transmitted?

A

By filariform larvae found in soil contaminated by human feces.

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33
Q

Describe the lifecycle of strongyloidiasis.

A

Penetration of skin –> migration hematogenously to the lungs –> enter alveoli and travel up bronchial tree to pharynx –> swallowed –> reach intestine –> develop into adults that lay eggs within the intestinal mucosa –> hatch into noninfectious larvae that migrate into the intestinal lumen to be excreted in stool.

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34
Q

What may occur due to autoinfection with strongyloides stercoralis?

A

Massive increase in worm burden and widespread dissemination of parasites throughout the body- hyperinfection with multiorgan dysfunction and septic shock.

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35
Q

How is strongyloidiasis diagnosed?

A

Rhabditform larvae in the stool with eggs and adult parasites seen on intestinal biopsy.

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36
Q

How is strongyloidiasis treated?

A

Ivermectin.

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37
Q

What virus is non enveloped and contains ss DNA?

A

Parvovirus.

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38
Q

What condition is most commonly caused by parvovirus?

A

Fifths Disease- erythema infectiosum, aplastic crises in sickle cell anemia, hydrops fetalis.

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39
Q

Name the three most common viruses responsible for bronchitis/bronchiolitis?

A

Influenza, RSV, Coronavirus.

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40
Q

What type of virus causes hep A?

A

Picornaviridae. (ssRNA)

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41
Q

What type of virus causes hep B?

A

Hepadnaviridae (dsDNA)

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42
Q

What type of virus causes hep C?

A

Flaviviridae (ssRNA)

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43
Q

What type of virus causes hep D?

A

Deltaviridae (ssRNA)

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44
Q

What type of virus causes hep E?

A

Hepeviridae (ssRNA).

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45
Q

What is the mechanism of injury inflicted by HBV on the liver?

A

Presence of viral HBsAg and HBcAg on the cell surface stimulates the host’s cytotoxic CD8+ T lymphocytes to destroy infected hepatocytes. No direct cytotoxic effect.

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46
Q

What is the most common cause of hepatic abscesses in underdeveloped and developed countries?

A

Underdeveloped- parasitic infection; developed- bacterial infection.

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47
Q

Name five routes by which pyogenic bacteria can gain access to the liver.

A

Biliary tract infection, portal vein pyemia, hepatic artery, direct invasion from an adjacent source, penetrating or trauma injury.

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48
Q

Infection with what pathogen is implicated by lancet shaped gram positive diplococci in a patient with fever and cough?

A

S. pneumoniae.

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49
Q

How is S. pneumoniae distinguished?

A

Gram positive, alpha hemolytic, optochin sensitive, bile sensitive diplococci.

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50
Q

C. tetani binds to receptors on presynaptic membranes of motor neurons and blocks release of what NTs?

A

Inhibitory NTs- glycine, GABA.

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51
Q

Antibodies to what organism are implicated in aggregation of the patients serum in a mixture of cardiolipin, cholesterol and lecithin?

A

Treponema pallidum.

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52
Q

Block of what three nerves provides complete perineal and genital anesthesia?

A

Pudendal, genitofemoral, ilioinguinal n.

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53
Q

Describe the histological appearance of hepatic tissue in A1AT deficiency.

A

Intracellular granules representing globules of unsecreted A1AT are seen within periportal hepatocytes; globules stain reddish pink with PAS stain and resist deigestion by diastase.

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54
Q

What is the lung pathology which creates dyspnea in patients with A1AT deficiency?

A

Interalveolar septa destruction.

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55
Q

Transmural inflammation of the arterial wall with fibrinoid necrosis is characteristic of what condition?

A

Polyarteritis nodosa.

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56
Q

Describe the inflammation seen in polyarteritis nodosa.

A

Segmental, transmural, necrotizing inflammation of medium to small sized arteries. The inflammation can occur in any organ except the lung.

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57
Q

What are the presenting symptoms/ signs of polyarteritis nodosa?

A

Fever, abdominal pain, peripheral neuropathy, weakness, weight loss.

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58
Q

What condition is associated with polyarteritis nodosa?

A

Hep B.

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59
Q

What is the triad of pre-eclampsia?

A

Hypertension, proteinuria, edema.

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60
Q

What is eclampsia?

A

Pre-eclampsia with seizures.

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61
Q

What is HELLP syndrome?

A

A progression of pre-eclampsia: hemolytic anemia, elevated liver enzymes, low platelets.

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62
Q

What type of bug is pseudomonas aeruginosa?

A

Non-lactose fermenting gram negative rod. It is oxidase positive.

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63
Q

What is a common cause of UTIs in patients with indwelling bladder catheters?

A

Pseudomonas aerguinosa.

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64
Q

What bug may cause transient bacteremia after dental procedures?

A

Viridans streptococci.

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65
Q

What is the mechanism of pathogenicity of viridans streptococci?

A

Production of extracellular polysaccharides (dextrans) via sucrose that facilitate adherence to fibrin and platelets which deposit at the site of endothelial trauma.

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66
Q

Describe the appearance of aspergillus fumigatus.

A

Thin, septate hyphae with acute V-shaped branching.

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67
Q

Name three conditions caused by aspergillus fumigatus.

A

Invasive aspergilosis in immunosuppressed patients, aspergillomas, allergic pulmonary aspergillosis.

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68
Q

What is the most common cause of aseptic meningitis?

A

Enterovirus.

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69
Q

Describe the structure of enteroviruses.

A

ssRNA viruses (cocksackie, echo, polioviruses)

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70
Q

How are enteroviruses transmitted?

A

Fecal-oral. They replicate in the GI tract.

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71
Q

In polio, damage to what part of the spinal cord causes hyporeflexic paralysis?

A

Anterior horn lower motor neuron cell bodies.

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72
Q

What symptoms occur first in polio?

A

Fever, malaise, aseptic meningitis

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73
Q

What is first line treatment for coagulase negative staphylococci (s. epidermidis) infection?

A

Empiric treatment with vancomycin (with or without rifampin or gentamicin)

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74
Q

In fifths disease, what sign often follows disappearance of facial rash?

A

Erythematous reticular, lace-like rash on the trunk and extremities

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75
Q

Where does parvovirus B19 predominately replicate?

A

In the bone marrow as it is highly tropic for erythroid precursor cells.

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76
Q

What is the most common cause of neonatal meningitis in the US?

A

Group B Strep. (followed by E. coli and listeria).

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77
Q

What are the two most common pathogens associated with meningitis in older infants (greater than 3 mos) and adults?

A

Strep pneumoniae, neisseria meningitidis.

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78
Q

What is the major virulence factor of E. coli strains that cause neonatal meningitis?

A

K1 capsular antigen

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79
Q

What is sporotrichosis?

A

A subcutaneous mycosis caused by Sporothrix schenckii.

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80
Q

What is sporothrix schenckii?

A

A dimorphic fungus found in the natural environment in the form of mold (hyphae) that resides on the bark of tree, shrubs, and garden plants.

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81
Q

How is sporothrix schenckii often transmitted and what are the clinical signs?

A

Thorn pricks (common in gardeners). It manifests as nodules that spread along lymphatics.

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82
Q

How do enterococci produce resistance to aminoglycosides?

A

Produce aminoglycoside-modifying enzymes that transfer different chemical groups (acetyl, adenyl, or phosphate) to aminoglycoside molecules which impairs antibiotic binding to ribosomal subunits.

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83
Q

What is the pathogenesis of staphylococcal scalded skin syndrome?

A

Exotoxin exfoliatin causes widespread epidermal sloughing, especially with gentle pressure (Nikolsky’s sign)

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84
Q

What types of viruses can induce viral protein synthesis in the host cell by acting as mRNA and using the host’s intracellular machinery for translation?

A

ssRNA, positive sense.

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85
Q

What type of virus is rhinovirus?

A

ssRNA positive sense.

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86
Q

Name four types of nonenveloped RNA viruses.

A

Enteroviruses, reoviruses, Norwalk virus, rhinovirus.

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87
Q

Name four types of enteroviruses.

A

Polio, cocksackie, echo, Hep A.

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88
Q

What nerve is most commonly injured in surgical procedures of the anterior neck and what symptoms result?

A

Recurrent laryngeal (travels close to the inferior thyroid artery); results in laryngeal muscle paralysis, hoarseness, dyspnea.

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89
Q

In pregnant women identified with group B strep colonization, what prophylactic measures should be performed?

A

Intrapartum antibiotics, usually with penicillin or ampicillin to prevent neonatal sepsis, pneumonia, and meningitis.

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90
Q

What is the presentation of coccidioides immitis in the lungs of patients?

A

As spherules in the lungs containing inhaled spores. These sphereules can rputure and disseminate to other organs and tissues.

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91
Q

Where is C. immitis endemic?

A

SW USA

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92
Q

How is C. immitis transmitted?

A

spore inhalation (spores are hyphae fragments)

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93
Q

What type of reaction is an acute hemolytic transfusion reaction?

A

Antibody mediated type II hypersensitivity reaction where anti ABO antibodies (mainly IgM) in the recipient bind corresponding antigens on transfused erythrocytes causing complement activation. Anaphylatoxins (C3a, C5a) cause vasodilation and symptoms of shock while formation of the MAC leads to complement mediated cell lysis.

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94
Q

What are the two most common causes of hematogenous osteomyelitis?

A

S. aureus followed by Strep pyogenes (group A strep)

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95
Q

How does tertiary syphilis affect the CV system?

A

Causes luetic aneurysms which begin with vasa vasorum endarteritis and obliteration, resulting in inflammation, ischemia, and weakening of the adventitia. Aneurysmal dilatation of the thoracic aorta can extend to dilate the aortic valve ring causing a murmur and mediastinal widening.

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96
Q

How does pregnancy increase liklihood of gallstones?

A

Estrogen induced cholesterol hypersecretion and progesterone induced gallblader hypomotility.

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97
Q

Cases of what four conditions are drastically reduced when H. influenzae vaccination is given?

A

Meningitis, pneumonia, sepsis, epiglottitis.

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98
Q

Name two conditions that can be caused by meningococcal sepsis.

A

DIC, hemorrhagic destruction of the bilateral adrenal glands (Waterhouse-Friderichsen syndrome).

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99
Q

What are ‘germ tubes’ and what species are they specific for?

A

True hyphae when incubated at 37degC for 3 hrs; specific for c. albicans.

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100
Q

Where is candida albicans a normal inhabitant?

A

GI tract (including oral cavity); it is thus present in sputum samples occasionally.

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101
Q

What is the most common cause of hydatid cysts?

A

Tapeworm- echinococcus granulosus

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102
Q

Where are echinococcus granulosas tapeworms endemic?

A

Mediterranean, Middle east, southern S. america, Iceland, australia, new zealand, southern africa OR SW USA with sheep and dog exposure.

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103
Q

What is the microscopic appearance of a hydatid cyst?

A

Encapsulated and calcified cyst (eggshell calcification) that contains fluid and budding cells and eventually may become daughter cysts. Outer cyst wall is composed of many layers of delicated, gelatinous sheets surrounded by a thick fibrous capsule.

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104
Q

What is the first line treatment plan for patients with hydatid cysts?

A

Surgery and adjunctive chemo with mebendazole or albendazole.

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105
Q

What is one complication associated with spilling hydatid cyst contents in the peritoneum?

A

Anaphylactic shock

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106
Q

Why is primaquine added to chloroquine for treatment of malaria?

A

To prevent disease relapse– it is used to completely eradicate organisms.

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107
Q

Which two bugs cause malaria?

A

Plasmodium vivax and plasmodium ovale.

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108
Q

What is one drawback to chloroquine treatment for plasmodia?

A

No activity against latent hepatic infections established by P. vivax and P. ovale; it is effective in eradicating chloroquine sensitive plasmodia from the bloodstream only.

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109
Q

What effect does ether and other organic solvents have on viral envelopes?

A

They dissolve the lipid bilayer that makes up the outer viral envelope. When dissolved, loss of infectivity occurs.

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110
Q

What kind of vaccine is the strep pneumo polysaccharide vaccine?

A

Unconjugated vaccine that induces a T cell independent response.

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111
Q

What is the difference between the polysaccharide and conjugate pneumococcal vaccines?

A

Pneumococcal conjugate- contains polysaccharide material attached to a protein antigen allowing for a robust T cell dependent response; polysaccharide only- no T cell response.

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112
Q

How does one catch Histoplasma capsulatum?

A

Present in bird and bat droppings (and is mold in soil) and is endemic to the Mississippi and Ohio River basins.

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113
Q

What is Histoplasma capsulatum?

A

Dimorphic fungus located intracellularly within macrophages that effects the lungs and reticuloendothelium system.

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114
Q

How does Histoplasma capsulatum present on light microscopy?

A

As small, intracellular oval bodies.

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115
Q

What are the symptoms of Histoplasma capsulatum?

A

Acute pulmonary disease (or chronic histoplasmosis if chronic lung disease was initially present– it appears like TB).

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116
Q

What is congenital torticollis?

A

Develops around 2-4 weeks of age and is caused by birth trauma or malposition of the head in utero –> head is tilted toward affected side with chin pointed away from contracture and soft tissue mass may be palpable in inferior one third of the affected SCM

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117
Q

How is congenital torticollis treated?

A

Conservative therapy and stretching exercises.

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118
Q

What signs/sx are characteristic of granulomatosis with polyangiitis (Wegener’s)?

A

Nasal mucosal ulcerations, glomerulonephritis (c-ANCA positive –> antibodies against neutrophil cytoplasmic bodies)

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119
Q

What is the mechanism of action of Foscarnet?

A

Pyrophosphate analog that does not require intracellular activation. It directly inhibits both DNA polymerase in herpesvirus and reverse transcriptase in HIV. It must be administered IV.

120
Q

What is Foscarnet most commonly used for?

A

Treating herpesvirus infections or ganciclovir-resistant CMV in patients with advanced AIDS.

121
Q

What is bacterial transformation?

A

Direct uptake of naked DNA from the environment by bacteria that are naturally competent

122
Q

Name three species of bacteria that participate in gene transfer by transformation.

A

Strep. pneumo, H. influenza, N. gonorrhoeae/meningitidis; this is caused by their capsular polysaccharides that allow for inhibition of phagocytosis.

123
Q

Ecthyma gangrenosum is strongly associated with what pathogen?

A

Bacteremia by P. aeruginosa

124
Q

Name four important virulence factor enzymes of P. aeruginosa that play a role in causing vascular destruction and cutaneous necrosis of ecthyma gangrenosum.

A

Exotoxin A (protein synthesis inhibition), elastase (degrades elastin), Phospholipase C (degrades cellular membranes), pyocyanin (generates reactive oxygen species)

125
Q

Name four subgroups of patients that are particularly susceptible to pseudomonas infections.

A

Neutropenic patients, hospitalized patients, patients with burns, patients with chronic indwelling catheters.

126
Q

Name two bugs that have toxins which inactivate EF-2 via ribosylation, thus inhibiting host cell protein synthesis (and causing cell death).

A

C. diptheriae, P. aeruginosa (diptheria toxin and exotoxin A respectively).

127
Q

Describe the clinical sequelae of typhoid fever.

A

Week 1: rising fever, bacteremia, relative bradycardia; week 2: abdominal pain, rose spots on trunk and abdomen; week 3: hepatosplenomeagly, intestinal bleeding and perforation.

128
Q

How is salmonella typhi spread?

A

Fecal oral following ingestion of contaminated food or water.

129
Q

How does salmonella penetrate the gut mucosa?

A

Via phagocytosis by M cells and bacteria mediated endocytosis by enterocytes; they then survive and proliferate within macrophages of peyers patches causing inflammation and bacteremia.

130
Q

Name the three most common pathogens (in order) responsible for secondary bacterial pneumonia following influenza infection.

A

Strep. pneumo, staph aureus, H. influenzae.

131
Q

What are the clinical features of Legionella pneumonia?

A

High fever, relative bradycardia, headache/ confusion, watery diarrhea, only mildly productive cough. MOST COMMONLY in SMOKERS. A common cause of CAP.

132
Q

What are the laboratory findings associated with Legionella pneumonia?

A

Hyponatremia, sputum gram stain with many neutrophils but few or no organisms

133
Q

How is Legionella pneumonia treated?

A

Respiratory fluoroquinolones or newer macrolides.

134
Q

How is Hep E spread?

A

Fecal-oral

135
Q

What indicates a positive monospot test?

A

Positive when horse RBCs are agglutinated in vitro by heterophil antibodies.

136
Q

What is cytochrome C?

A

A mitochondial enzyme that activates caspases and indirectly brings about cell death through intrinisc pathway apoptosis.

137
Q

What causes hot tub folliculitis and how does it present?

A

P. aeruginosa; it occurs in minor outbreaks following exposure to a pool or spa. Presents as a diffuse, pruitic papulopustular rash.

138
Q

What type of bug is pseudomonas aerguinosa?

A

Gram negative, oxidase positive, non-lactose fermenting, motile rods that produce pigment.

139
Q

What is the most common cause of death in patients with diptheria?

A

cardiomyopathy.

140
Q

In a patient with HIV, what do high rates of Pol gene mutations signify?

A

Acquired resistance to HIV reverse transcriptase inhibitors and HIV protease inhibitors.

141
Q

In a patient with HIV, what do high rates of Env gene mutations signify?

A

Enabled escape from host neutralizing antibodies.

142
Q

What is the most common cause of acute hemorrhagic cystitis in children?

A

Adenovirus (esp serotypes 11 and 21)

143
Q

What is the appearance of corynebacterium diptheriae?

A

Found in clumps resembling Chinese letters or joined in V or Y shaped chains on microscopic examination. Cytoplasm contains metachromatic granules that stain with aniline dyes like methylene blue.

144
Q

Describe the toxin of C. diptheriae?

A

Two subunit AB exotoxin. B subunit binds to heparin-binding epidermal growth factor receptor found on cardiac and neural cells. It induces endocytosis of the toxin and the A subunit inhibits host cell protein synthesis by catalyzing the ADP ribosulation of protein elongation factor EF-2.

145
Q

How does diptheria exotoxin inhibit host cell protein synthesis?

A

By catalyzing the ADP ribosulation of host cell elongation factor 2.

146
Q

What condition is implied by impaired sperm motility due to abnormal tail function?

A

Primary ciliary dyskinesia– Defect in axoneme of sperm flagellum; many have Kartageners.

147
Q

Describe the sequence of replication of the hepatitis B virus.

A

dsDNA –> template + RNA –> progeny double stranded DNA

148
Q

Describe the HBV genome

A

Partially double stranded circular DNA molecule

149
Q

What is enterobiasis and what is the etiologic organism?

A

Enterobiasis; etiologic organism is the pinworm.

150
Q

How does enterobius vermicularis infection present?

A

Perianal pruritus; dx via scotch tape test.

151
Q

What is first line treatment for enterobius vermicularis?

A

Albendazole. (or pyrantel pamoate in pregnant patients)

152
Q

What is the mechanism of action of zidovudine (AZT)?

A

Nucleoside analog, retroviral reverse transcriptase inhibitor

153
Q

In what condition is postauricular and occipital lymphadenopathy particularly common?

A

Rubella (a togavirus)

154
Q

What is the pattern of spread of ruboela (measles) or rubella?

A

Maculopapular rash that begins on the face and spreads to the trunk and extremities.

155
Q

What bug causes bacterial vaginosis?

A

Gardnerella vaginalis.

156
Q

What test is diagnostic for bacterial vaginosis?

A

KOH addition for a whiff test; clue cells on wet mount

157
Q

What discharge is typical of BV? Trichomonas?

A

BV- gray discharge with fishy odor; Trichomonas- yellow green, foamy discharge.

158
Q

In what way does the hepatitis D virus need Hep B to survive?

A

Hep D requires coating of its virus by hep B before it can infect hepatocytes and multiply.

159
Q

What findings are characteristic of aspergillus fumigatus derived allergic bronchopulmonary aspergillosis?

A

High serum IgE levels, eoinophilia, IgE and IgG serum antibodies to Aspergillus. Intense airway inflammation, mucus plugging with exascerbations and remissions. Repeated exascerbations may cause transient pulmonary infiltrates and proximal bronchiectasis. It is often a complication of asthma.

160
Q

Describe the toxin associated with EHEC O157:H7

A

Shiga like toxin- inactivates the 60S ribosomal subunit inhibiting protein synthesis and causing cell death Shiga like toxin that is unable to ferment sorbitol and does not produce a glucuronidase.

161
Q

Describe the properties of EHEC O157:H7.

A

Unable to ferment sorbitol, does not produce a glucuronidase.

162
Q

What is the significance of mycobacteria growing as serpentine cords on enriched media?

A

Contains presence of cord factor, a mycoside which establishes virulence through neutrophil inhibition, mitochondrial destruction, release of TNF.

163
Q

What is the recommended treatment for cryptococcal meningitis?

A

amphotericin B and flucytosine followed by long term fluconazole.

164
Q

Why are pulmonary infarcts almost always hemorrhagic?

A

Due to the dual blood supply of the lungs (pulmonary and bronchial arteries)

165
Q

What are the two most common causes of tricuspid endocarditis in IV drug users?

A

S. aureus, followed by P. aeruginosa.

166
Q

What is the mechanism of the toxin of C. diff?

A

Toxin A- enterotoxin, toxin B: cytotoxin Both inactivate Rho-regulatory proteins involved in signal transduction and actin cytoskeletal structure maintenance (disruption of actin cytoskeletal structure). They disrupt intercellular tight junctions causing cell rounding/retraction and increased intestinal fluid secretion. They both have inflammatory effects and can induce apoptosis.

167
Q

What is a side effect of topical a-adrenergic agonist decongestant overuse?

A

Negative feedback causing decreased NE synthesis and release from nerve endings which decreases their effect.

168
Q

Name three drugs that can be used to treat C. Diff infection.

A

Oral metronidazole, vancomycin, fidaxomicin.

169
Q

When is fidaxomicin used to treat c. diff?

A

In patients with recurrent colitis or at increased risk of recurrence.

170
Q

What is the mechanism of action of Fidaxomicin?

A

Inhibits the sigma subunit of RNA polymerase to impair protein synthesis and cause cell death. It is an oral drug with minimal systemic absorption and a narrow spectrum and has less effect on colonic flora than metronidazole or vancomycin.

171
Q

Name the two genuses of bacteria that form spores.

A

Bacillus and Clostridium (they can survive boiling)

172
Q

Describe the typical appearance of blastomyces dermatidis.

A

Encapsulated round yeast with a single, broad based bud and doubly refractive walls. Dimorphic fungus.

173
Q

Where is blastomyces dermatidis endemic?

A

Great Lakes, Ohio and Mississippi river regions.

174
Q

What organ is the primary site of involvement of blastomyces dermatidis?

A

Lungs (contracted due to inhalation of aerosolized fungus from the environment).

175
Q

What nerve provides somatic sensory and taste innervation to the the tongue?

A

Branch of the mandibular division of the trigeminal nerve; taste provided by chorda tympani of CN VII. Posterior portion of the tongue is innervated entirely by CN IX.

176
Q

What toxin is associated with ETEC?

A

Heat labile Cholera toxin and heat stable toxin

177
Q

What is the mechanism of heat labile toxin?

A

Activation of adenylate cyclase to cause increased cAMP.

178
Q

What is the mechanism of heat stable enterotoxin?

A

Activation of guanate cyclase leading to increased intracellular cGMP

179
Q

What is considered the trigger of neoplastic change in HBV induced HCC?

A

Integration of viral DNA into the genome of host hepatocytes.

180
Q

What is the presence of Giardia lamblia on iodine stained stool smear?

A

Ellipsoidal cysts with smooth, well defined walls and 2+ nuclei

181
Q

How is Giardia lamblia contracted and how is it treated?

A

Most common cause of diarrhea in campers/ hikers caused by drinking contaminated water. Tx with metronidazole.

182
Q

How do viruses that use eukaryotic ribosomes for protein synthesis convert their polycistronic genome into monocistronic mRNA?

A

By production of a polyprotein product from a single mRNA transcript. This is later cleaved by a viral protease to generate a set of functional, individual viral proteins. Viruses with ss, positive sense, linear, nonsegmented RNA viruses use this method.

183
Q

How do humans acquire schistosomiasis?

A

Via contact with freshwater that contains snails infected with Schistosoma larvae.

184
Q

What is the difference in conditions caused by S. japonicum, S. mansoni, and S. haematobium?

A

S. japonicum and S. mansoni cause intestinal and hepatic schistosomiasis and S. haematobium causes urinary schistosomiasis.

185
Q

What condition often proceeds scarlet fever?

A

Streptococcal pharyngitis.

186
Q

Describe the oral findings associated with scarlet fever.

A

Erythematous, swollen tonsils possibly covered with gray-white exudates. Tongue may have inflammed red papillae with an appearance of a red strawberry. Rash then appears on the body

187
Q

What complications are associated with scarlet fever?

A

acute rheumatic fever, glomerulonephritis.

188
Q

Name five catalase positive organisms.

A

Staphylococcus aureus, Burkholderia cepacia, Serratia marcescens, Nocardia, Aspergillus.

189
Q

What two factors are required by H. influenzae to grow on blood agar?

A

Factor X (exogenous hematin) and factor V (NAD+)

190
Q

What is the satellite phenomenon of H. influenzae growth?

A

Growth of H. influenzae in the presence of S. aureus on blood agar where H. influenzae grows near the beta hemolytic S. aureus colonies because they produce needed factor X and V (NAD+)

191
Q

What pathogen causes “woolsorters disease”?

A

Pulmonary anthrax.

192
Q

What causes the pathogenicity of Bacillus anthracis?

A

Antiphagocytic capsule containing D-glutamate instead of a polysaccharide.

193
Q

What is an important symptom of pulmonary anthrax?

A

Hemorrhagic mediastinitis (widened mediastinum).

194
Q

How does bacillus anthracis appear on microscopy?

A

As long chains “serpentine or medusa head” appearance

195
Q

What gram positive rod produces a narrow zone of beta hemolysis on blood agar?

A

Listeria monocytogenes (resembles streptococcus agalactiae)

196
Q

What fact identifies listeria isolates?

A

Tumbling motility at 22 degrees C and multiplication at very low temperatures.

197
Q

How does listeria manage to survive intracellularly?

A

Through listeriolysin O, a pore forming toxin that is selectively activated within acidified phagosomes.

198
Q

What type of immunity is necessary for elimination of intracellular bacteria?

A

Cell mediated immunity.

199
Q

Name the five clinically relevant species of dimorphic fungi.

A

Sporothrix schenckii, Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatidis, Paracoccidioides brasillensis.

200
Q

How do dimorphic fungi appear on microscopic KOH presentation?

A

As large yeast cells with a single bud.

201
Q

What is the purpose of an ecological study?

A

To analyze populations (not individuals) in terms of the frequency of a given characteristic and a given outcome. (Cross sectional surveys analyze INDIVIDUALS).

202
Q

What is colonizing aspergillus?

A

Fungus ball inside an old lung cavity. Aspergillus does not invade the lung tissue but instead grows inside the cavity.

203
Q

What patients are most susceptible to enterococcal endocarditis?

A

Elderly men who have recently undergone manipulation of areas colonized by enterococcus (GI, GU tracts), women following obstetrical procedures.

204
Q

Describe the bacterial characteristics of Enterococci.

A

Lancefield group D streptococci; can be cultured in 6.5% saline and bile. Gamma hemolytic.

205
Q

Low levels of C1 esterase inhibitor are diagnostic of what condition?

A

Hereditary angioedema (AD condition causing painless, non-pitting, well circumscribed edema). This causes increased bradykinin activity.

206
Q

What drugs are contraindicated in a patient with low levels of C1 esterase inhibitor?

A

ACE-inhibitors.

207
Q

What is the immune function of the spleen?

A

Antibody synthesis and a reservoir of phagocytic cells capable of removing circulating pathogens.

208
Q

What is the mechanism of pathogenicity of Rhabdoviridae?

A

ssRNA virus enveloped by a bullet shaped capsule which is studded by glycoprotein spikes that bind to nicotinic ACh receptors. They bind to ACh receptors on peripheral nerve axons and travel reterograde to the CNS.

209
Q

What cellular receptor is associated with CMV?

A

Cellular integrins

210
Q

What cellular receptor is associated with EBV?

A

CR2 (CD21)

211
Q

What cellular receptor is associated with HIV?

A

CD4 and CXCR4/CCR5

212
Q

What cellular receptor is associated with rabies virus?

A

Nicotinic ACh receptor

213
Q

What cellular receptor is associated with Rhinovirus?

A

ICAM1 (CD54)

214
Q

Which picornavirus is the most acid labile?

A

Rhinovirus (cant colonize GI tract or cause gastroenteritis).

215
Q

What bug causes scabies?

A

Sarcoptes scabiei mites

216
Q

Skin scrapings of scabies scabs will show ___.

A

Mites, ova and feces.

217
Q

What is the only pathogenic fungus with a polysaccharide capsule?

A

Cryptococcus neoformans. It is a major virulence factor.

218
Q

What stain is used to visualize cryptococcus neoformans?

A

Mucicarmine stain or Methenamine silver (GMS)

219
Q

Superantigens interact and stimulate which immunologic cells?

A

MHC molecules on APCs in the variable region of the T lymphocyte receptor to cause widespread activation of T cells. This causes release of IL-2 from T cells and IL-1 and TNF from macrophages.

220
Q

The toxin of bacillus anthracis most resembles the toxin produced by what bacteria?

A

B. pertussis. (Both function to increase cAMP levels cuasing edema and phagocyte dysfunction)

221
Q

What supplementation do Mycoplasma species require to grow on artificial media?

A

Cholesterol.

222
Q

How does N. Meningitidis gain access to the CNS?

A

Nasopharyx –> invasion of mucosal epithelium –> bloodstream –> choroid plexus –> through BBB to CNS

223
Q

How does H. influenzae enter the CNS?

A

Pharynx –> lymphatics –> meninges.

224
Q

What Rx is used for erythrocytic forms of Plasmodia (malaria) infection?

A

Chloroquine or mefloquine.

225
Q

Primaquine is required to kill which plasmodia species?

A

P. vivax and P. ovale in liver schizonts.

226
Q

Which cephalosporins have good anti-pseudomonal coverage?

A

Cefepime and ceftazidime.

227
Q

Which cephalosporin has poor pseudomonal coverage?

A

Ceftriaxone.

228
Q

How does the occurance of rheumatic fever differ between streptococcal pharyngitis infections and impetigo?

A

Strep pharyngitis is associated with rheumatic fever. PSGN can follow either; skin infection is not associated with rheumatic fever.

229
Q

What is first line treatment of toxoplasmosis gondii in an HIV positive patient?

A

Pyrimethamine and sulfadiazine.

230
Q

What characteristic radiographic findings indicate toxoplasmosis in an AIDS patient?

A

Ring enhancing lesions in both cerebral hemispheres.

231
Q

Primary CNS lymphoma in an immunocompromised patient requires what type of immune response?

A

B-lymphocytic

232
Q

What sequelae are characteristic of rubella in a pregnant woman and her fetus?

A

Mother: polyarthralgia; fetus: deafness, cataracts, cardiac malformations (ex. PDA)

233
Q

What is the most common cause of bacterial meningitis in adults of all ages?

A

Strep pneumo.

234
Q

How does strep pneumo appear on gram stain?

A

As lancet shaped gram positive cocci in pairs.

235
Q

What side effects are associated with protease inhibitors?

A

Hyperglycemia, lipodystrophy, DDIs due to inhibition of p450.

236
Q

On what media is C. diptheriae cultured?

A

Cysteine-tellurite agar- colonies are black in colour.

237
Q

How does C. diptheriae appear on microscopy after methylene blue staining?

A

Shows intracellular polyphosphate granules (metachromatic granules)

238
Q

What is bordet-gengou medium used to culture?

A

Bordetella pertussis.

239
Q

Name three characteristic findings associated with tertiary syphilis.

A

Neurosyphilis, CV involvement, gummas (necrotizing granulomas occurring on skin, mucosa, subcutaneous tissue, bones and within organs).

240
Q

Describe the appearance of CMV on lung biopsy.

A

Enlarged, centrally located epithelial cell with intranuclear and cytoplasmic inclusions.

241
Q

Name two ways in which campylobacter infection can be acquired.

A

Domestic animals (cattle, chicken, dogs) or from contaminated food.

242
Q

How is vibrio parahemolyticus transmitted?

A

Consumption of contaminated shellfish.

243
Q

A therepeutic drug that is a monoclonal anti-CD21 antibody could interefere with the attachment of what virus to cells?

A

EBV (also attaches to gp350)

244
Q

What cellular mediator is associated with Parvovirus B19?

A

Erythrocyte P antigen.

245
Q

What is another name for colistin?

A

polymyxin (i.e. on Thayer Martin agar to inhibit gram negatives)

246
Q

What is the cause of gram negative sepsis?

A

Release of LPS from bacterial cells during division or by bacteriolysis (not through secretion). This leads to widespread release of IL-1 and TNF-a which causes sx of septic shock. It is caused by LIPID A.

247
Q

What bug causes Chagas disease?

A

Trypanosoma cruzi

248
Q

Name three symptoms of Chagas disease.

A

Achalasia, megacolon, megaureter.

249
Q

What is the major virulence factor of Strep pyogenes and how does it work?

A

Protein M; it inhibits phagocytosis and complement activation, mediates bacterial adherence, and is the target of humoral immunity to S. pyogenes.

250
Q

What type of drug is acyclovir?

A

Guanosine analogue.

251
Q

Why is acyclovir not used to treat EBV and CMV?

A

It requries monophosphorylation by viral thymidine kinase to active form. EBV and CMV do not produce the same thymidine kinase that activates this.

252
Q

In a diabetic patient with facial pain, headache, and a black necrotic eschar in the nasal cavity, what condition is indicated?

A

Mucormycosis (Rhizopus and Absidia)

253
Q

How is mucomycosis diagnosed and what is its treatment?

A

Dx by histological examination of affected tissue (non-septate hyphae with right angle branching). Tx is surgical debridement and amphotericin B.

254
Q

Measurements of what hormone should be monitored in a patient following evacuation of a hydatidiform mole?

A

b-hCG to rule out invasive mole or choriocarcinoma.

255
Q

What is the most common cause of opportunistic mycosis?

A

Candida albicans

256
Q

What tests are diagnostic for candida albicans?

A

Yeasts and pseudohyphae on light microscopy and positive germ tube test.

257
Q

What immune response is compromised by silicosis?

A

Disruption of macrophage phagolysosomes by internalized silica particles. This impairs killing of intracellular mycobacteria.

258
Q

What is another name for fifth disease and what causes it?

A

Erythema infectiosum; caused by parvovirus B19 (ssDNA virus).

259
Q

What is the classic presentation of roseola infantum?

A

High fever and maculopapular rash that starts on the trunk and spreads peripherally. (HSV-6)

260
Q

What bug causes epiglottitis?

A

H. influenzae type B.

261
Q

What does the Hib vaccine include?

A

Polyribosyl-ribitol phosphate (PRP) component of h. influenzae capsule conjugated with diptheria or tetanus toxoid.

262
Q

What prevents C. diff infection?

A

Normal intestinal bacteria suppress overgrowth.

263
Q

Name five key symptoms of Legionella pneumonia.

A

High fever with relative bradycardia, headache/confusion, watery diarrhea, HYPONATREMIA, Sputum stain showing many neutrophils but few or no organisms. It presents as a unilobar infiltrate that progresses to consolidation.

264
Q

What viral factor determines the range of host infectivity?

A

Surface glycoproteins

265
Q

Name four gram negative rod lactose non fermenters and describe how they can be distinguished.

A
  1. P. aerguinosa (oxidase positive)
  2. Shigella (ox. negative, no H2S production on TSI agar)
  3. Salmonella, Proteus (ox. negative, H2S production)
266
Q

What is the essential pathogenic mechanism for shigella infection?

A

Mucosal invasion (gains access to gut epithelium by entering M cells in Peyer’s patches)

267
Q

What is the main toxin produced by C. perfringens and how does it work?

A

Lecithinase (alpha toxin) degrades lecithin, a component of cellular phospholipid membranes causing membrane destruction, cell death, and widespread necrosis and hemolysis.

268
Q

How are Actinomyces israelli abscesses treated?

A

Long term Penicillin tx and surgical debridement

269
Q

How do listeria bacteria gain access to the bloodstream in Listeria meningitis?

A

Following ingestion of contaminated food (unpasturized milk, undercooked meats, unwashed raw vegetables). Opportunistic infection.

270
Q

What is a unique feature of listeria?

A

Tumbling motility.

271
Q

How do mucormycoses appear on light microscopy?

A

Mold form with broad nonseptate hyphae that branch at wide (often 90 deg) angles

272
Q

What type of virus is parvovirus?

A

non-enveloped ssDNA virus

273
Q

In an immunocompromised patient with cough, fever and hepatosplenomegaly, what bug that forms ovoid bodies within a macrophage causes this condition?

A

Histoplasma capsulatum.

274
Q

How does histoplasmosis appear on CXR?

A

Diffuse pulmonary infiltrates with hilar adenopathy. Cavitary lesions in upper lung loves, calcified nodes and fibrotic scarring.

275
Q

How do the laboratory markers of liver injury appear in a neonate who acquired Hep B (with HBeAg)?

A

mildly elevated

276
Q

What is the most common cause of croup?

A

Parainfluenza virus (a paramyxovirus)

277
Q

Name 3 symptoms of croup.

A

Laryngotracheitis- brassy, barking cough, dyspnea, recent hx of URI.

278
Q

On stool sample of a patient with vibrio cholerae or E. coli, what is seen?

A

No erythrocytes or leukocytes (no cell death); mucous and sloughed epithelial cells only. No inflammatory cells.

279
Q

How does diptheriae toxoid prevent disease?

A

It induces production of circulating IgG against exotoxin B subunit.

280
Q

What are the two most serious sequlae of diptheria?

A

Neurotoxicity, cardiac toxicity.

281
Q

What type of bacteria is Pseudomonas aerguinosa?

A

Nonlactose fermenting, oxidase positive, motile, gram negative rod.

282
Q

What test is used to diagnose cryptococcus neoformans meningoencephalitis in HIV + patients?

A

Latex agglutination test detects polysaccharide capsule antigen. India ink staining of CSF shows round or oval budding yeast.

283
Q

What is blastomycosis dermatitidis?

A

Dimorphic fungi that can cause pulmonary infections in immunocompetent people (endemic to Great lakes, MS and OH river basins). Transmitted via respiratory route. May cause a chroinc pneumonia characterized by granuloma formaiton.

284
Q

How is blastomycosis dermatitidis diagnosed?

A

Sputum stain with KOH- thick doubly refractive walls and each yeast has a broad based bud.

285
Q

How is blastomycosis dermatidis treated?

A

Itraconazole.

286
Q

What type of bacteria is Klebsiella?

A

Encapsulated lactose fermenting gram neg. bacillus that appears mucoid in culture.

287
Q

Spherules filled with small round endospores from lung biopsy indicate infection with what pathogen?

A

Coccidioides immitis.

288
Q

How does coccidiodes immitis appear on silver stain?

A

Thick walled spherules packed with endospores.

289
Q

What type of vaccine is the rubella vaccine?

A

Live attenuated vaccine

290
Q

What type of viruses are responsible for most cases of aseptic meningitis in children?

A

Enteroviruses (coxsackie virus, echovirus, poliovirus, enterovirus)

291
Q

What is seen in the CSF of a patient with viral meningitis?

A

Lymphocytic pleocytosis, modestly elevated protein, normal glucose

292
Q

What is a possible systemic manifestation of M. pneumonia infection?

A

Hemolysis due to antigenic similarity between antigens in the cell membrane of M. pneumoniae and in the cell membrane of erythrocytes (“cold agglutins”). Stevens Johnson syndrome and joint pains are also possible.

293
Q

What is responsible for the toxic effects observed in N. meningitidis infections and meningococcemia?

A

Meningococcal lipopligosaccharide (LOS). Blood levels of LOS correlate closely with morbidity and mortality.

294
Q

How can non-pathogenic Cornyebacterium cause severe pseudomembranous pharyngitis?

A

By acquiring the Tox gene via lysogenization by a temperate bacteriophage.

295
Q

What type of prognosis is required for referring a patient for hospice care?

A

Survival prognosis of less than 6 months.

USMLE World Missed Questions (32 decks)

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