Micro USMLE 8-29(17) (qmax 8/24 1-31)

Question 1

Based on the patient’s fever and the presence of erythema migrans, it’s likely she has Lyme disease. The treatment of choice for an adult would be doxycycline, a tetracycline antibiotic used to treat bacterial infections. However, the use of doxycycline in children can cause?

Answer 1

discoloration of teeth.

Lyme disease initially manifests with a rash that surrounds the bite site of the Ixodes tick, the organism that is responsible for transmitting the Borrelia burgdorferi spirochete. There are three stages of Lyme disease:

Stage 1, known as erythema chronicum migrans (which is described in the stem of the question) manifests with a rash, fevers, chills, fatigue, and malaise. This patient is currently in stage 1 of the disease.
Stage 2 manifests with intermittent joint pain, neurologic abnormalities (facial nerve palsy), and cardiac abnormalities (heart block), which occur 4-6 weeks after primary infection.
Stage 3 occurs months to years after the initial infection and manifests as arthritis, synovitis, or subacute encephalitis.

Question 2

Doxycycline is the treatment of choice for patients with stage 1 Lyme disease. Doxycycline acts by inhibiting attachment of aminoacyl-transfer RNA in bacteria. Its adverse effects include gastrointestinal distress, photosensitivity, discoloration of teeth in children under 8 years of age, and inhibited bone growth in children. While it is not contraindicated, children younger than 8 years old should use doxycycline only in cases of severe or life-threatening conditions (eg, anthrax, Rocky Mountain spotted fever). Other toxicities include?

Answer 2

liver toxicity and nephrotoxicity. Because this patient is allergic to amoxicillin, alternative treatments would be cefuroxime or azithromycin.

Here are the agents related to the other listed side effects:

Gray baby syndrome can occur due to insufficient metabolism of chloramphenicol, a drug which is infrequently used in the United States due to the variety of antibiotics available.
Ototoxicity can occur with aminoglycoside use, but these drugs are not indicated for treatment of Lyme disease.
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Question 3

Red person syndrome occurs when vancomycin is rapidly infused beyond 1 g per hour and is histamine related, but vancomycin is not indicated to treat ?

Answer 3

Lyme disease.
An odd metallic taste can occur with metronidazole use. Metronidazole is used to treat anaerobic infections such as Bacteroides, Clostridium, or H. pylori by forming toxic metabolites that damage DNA in bacterial cells.

Question 4

Lyme disease is caused by the Borrelia burgdorferi spirochete transmitted by the Ixodes tick. It initially manifests with a fever and a rash (erythema chronicum migrans), and, if left untreated, can involve the central nervous system and the joints. While the treatment of choice for adults is doxycycline, its use in children can lead to such adverse effects as ?

Answer 4

discoloration of teeth and inhibited bone growth. Amoxicillin or cefuroxime axetil are prescribed for children under age 8 years and women who are nursing or pregnant.

Question 5

This patient, who has symptoms of renal obstruction and recurrent urinary tract infections (UTIs), suffers from a staghorn calculus—the large radiopaque object in the circle. This diagnosis is supported by the large branching stone that fills the renal pelvis and calyces on plain film. Struvite stones are composed of ammonium magnesium phosphate (struvite) and are caused by infection by urease-producing microorganisms, including?

Answer 5

Proteus species (most common), Staphylococcus species, Ureaplasma, and Klebsiella species. Urease breaks down urinary urea into ammonia plus carbon dioxide. The increased ammonia combines with water to increase availability of ammonium in the alkaline urine, which will increase struvite stone formation. Patients can present with a UTI, mild flank pain, or hematuria. On urinary analysis, they typically have alkaline urine pH (>7.0), often with multiple magnesium ammonium phosphate crystals in the urine sediment. Staphylococcus saprophyticus is the second most common cause of UTIs (behind E. coli) and is urease positive.

Question 6

Struvite stones are caused by infection by?

Answer 6

urease-producing microorganisms, most commonly, Proteus, Klebsiella, and Staphylococcus.

Question 7

Acinetobacter is a gram-negative coccobacillus that has emerged from an organism of questionable pathogenicity to an infectious agent of importance to hospitals worldwide. This bacterium has the ability to develop resistance through several different mechanism, leading to drug-resistant strains. Healthcare exposures, including prior antibiotic receipt are associated with colonization and infection by drug-resistant isolates. Also, Acinetobacter does not produce urease and would not be able to produce a struvite stone. This patient has a struvite stone and does not have healthcare exposures so Acinetobacter is not the correct answer.

Escherichia coli is a gram-negative, fast lactose fermenter rod. E. coli is the most common cause of UTIs. The virulence factor responsible for the UTIs is ?

Answer 7

the microorganism’s fimbriae. E. coli is not the correct answer to this question because this microorganism does not produce urease and will not form staghorn calculi.

Pseudomonas is a nonfermentative gram-negative aerobic rod that is ubiquitous in the environment and grows easily on a variety of media. This bacterium also is distinguishable from other gram-negative organisms because it produces oxidase. Pseudomonas may cause wound and burn infections, pneumonia in cystic fibrosis patients, external otitis, diabetic osteomyelitis and UTIs. This is not the correct answer to this question because Pseudomonas does not produce urease and thus would not produce a staghorn calculi.

Question 8

Citrobacter bacteria are gram-negative, facultative anaerobic rods or coccobacilli. Citrobacter are opportunistic nosocomial pathogens that may cause ?

Answer 8

complicated UTIs. This is not the correct answer for this question because this patient has a struvite stone. Citrobacter does not produce urease so it would not be able to produce these symptoms in this patient.

Question 9

This patient has a high fever, back pain, hepatomegaly, jaundice, and black vomitus—all suggestive of the diagnosis of yellow fever virus. Diagnosis can be confirmed by testing with reverse-transcriptase polymerase chain reaction within 6 to 10 days. A liver biopsy specimen may show signs of Councilman bodies (acidophilic inclusions in the liver); however, because of the bleeding tendency of patients with yellow fever, liver biopsy is generally not recommended and only advisable at autopsy to confirm the cause of death.

Yellow fever virus is spread by mosquitoes in endemic areas (eg, South America, Africa). Preventive treatment includes the use of live-attenuated vaccines before travel to these areas.

Yellow fever virus belongs to the viral family Flaviviridae, a family of linear, single-stranded, enveloped, positive RNA viruses with icosahedral capsids. Of the viruses listed, only ?

Answer 9

the hepatitis C virus is another member of the family Flaviviridae. Dengue, West Nile virus, St. Louis encephalitis, and Zika virus are other flaviviruses.

Question 10

Chikungunya is in the family Togaviridae and causes Chikungunya fever. Lassa virus is in the family Arenaviridae and causes Lassa fever. Coronavirus is in the family Coronaviridae and can cause?

Answer 10

fever, a runny nose, cough, and sore throat. Influenza virus is in the family Orthomyxoviridae; common symptoms of “the flu” include fever, chills, muscle aches, cough, congestion, runny nose, headaches, and fatigue.

Question 11

Yellow fever is a mosquito-borne viral illness caused by a flavivirus, a member of the Flaviridae family, which includes single-stranded, positive, linear RNA viruses. The Flaviviridae family also includes?

Answer 11

hepatitis C virus, Dengue, West Nile virus, St. Louis encephalitis, and Zika virus.

Question 12

his patient presents after a recent penetrating injury with edema, erythema, and soft tissue crepitus. This is highly suggestive of a skin infection with Clostridium perfringens. C. perfringens is an ?

Answer 12

obligate anaerobic rod that produces a-toxin, a phospholipase (specifically lecithinase) that cleaves lecithin in the plasma membrane of cells. This toxin causes myonecrosis of the soft tissue, an accumulation of subcutaneous gas, and hemolysis (double zone of hemolysis on blood agar). Accumulation of subcutaneous gas causes the classic crepitus, a crackling sound heard on palpation. C. perfringens can also cause cellulitis by the same mechanism and food poisoning by releasing its enterotoxin within the gastrointestinal tract and subsequently causing watery diarrhea.

Question 13

Obligate anaerobic rod that produces a cytotoxin describes C. difficile, which causes diarrhea and colitis.
Dimorphic fungus that appears as cigar-shaped, budding yeast describes Sporothrix schenckii, which causes a local pustule or ulcer with ascending lymphangitis.
Facultative anaerobic rod with a protein capsule describes Bacillus anthracis, which causes?

Answer 13

cutaneous or pulmonary anthrax.
Obligate anaerobic rod that produces an exotoxin describes C. tetani, which causes tetanus with symptoms of spastic paralysis, trismus, risus sardonicus, and opisthotonos.

Question 14

C. perfringens is an obligate anaerobic rod that produces α-toxin (lecithinase), which cleaves lecithin, a component of the plasma membrane. Production of lecithinase causes?

Answer 14

cellulitis, myonecrosis (leading to crepitus), and hemolysis.

Question 15

In this experiment, previously antibiotic-sensitive bacteria are found to be resistant when grown in the presence of another bacterial species that shows antibiotic resistance. The process of genetic transfer described is ?

Answer 15

bacterial conjugation, which refers to the process by which DNA is transferred from one bacterium to another via a pilus, or hair-like structure. It is a common mechanism by which bacteria share the necessary genes to confer antibiotic resistance. It involves prokaryotic cells and the transfer of plasmid DNA

Question 16

The other answer options represent other modes of genetic transfer between bacterial cells and modes of genetic diversity. Although these alternative modes of DNA transfer may result in antibiotic resistance, the use of the pilus structure between donor and recipient bacterial cells is representative of the conjugation mode of gene transfer.

Hybridization refers to the act of mixing different species or varieties of animals or plants.
Transduction involves DNA transfer from infection of a virion, which is packaged with DNA from another bacterial cell.
Transformation is?

Answer 16

a process through which naked purified DNA (DNA outside any cell exposed in the environment) is taken up by bacteria and incorporated into the chromosome.
Translocation describes the transfer of DNA from one part of a chromosome to a new position on the same chromosome or to a different chromosome.

Question 17

Bacterial conjugation refers to?

Answer 17

the direct transfer of DNA, via a bacterial pilus, from one bacterium to another. It is a common mechanism by which bacteria share the necessary genes to confer antibiotic resistance.

Question 18

This patient’s urinary frequency, dysuria, and history of recurrent urinary tract infections (UTIs) suggest that she has an active UTI. It is reasonable to suspect that it is caused by Escherichia coli, the most common pathogen in UTIs in young, sexually active females. E coli is a gram-negative rod that?

Answer 18

ferments lactose, forming pink colonies on MacConkey agar.
Risk factors for recurrent UTIs include increased frequency of sexual intercourse, use of spermicide, and loss of estrogen effect on the vagina and periurethral structures. Patients can be advised to urinate after sexual intercourse to prevent contamination of the urethra. Cranberry juice or cranberry tablets may provide a protective effect, especially in women with recurrent UTIs.

Question 19

The other characteristics listed are not typical of E coli. Growth at 42°C is characteristic of Campylobacter jejuni. Bile insensitivity and variable hemolysis are characteristics of Enterococcus. α-Hemolysis is characteristic of Streptococcus pneumoniae. Dark-field microscopy is used to visualize?

Answer 19

Treponema pallidum. Oxidase positivity and maltose fermentation are characteristic of Neisseria meningitidis.

E coli, which ferments lactose, is the most common cause of urinary tract infections in sexually active young women.

Question 20

This patient presents with decreased oral intake, increased muscle tone, hyperactive reflexes, and a seizure followed by arching of the back (opisthotonos). This is concerning for tetanus infection, especially in the setting of a likely infection of the umbilical stump. Although exceedingly rare in developed countries, neonatal tetanus is still a prominent disease in the developing world, where mothers are not vaccinated (cannot confer immunity to their infants) and the umbilical stump—which is the site of the actual infection—is not kept hygienic. Other classic symptoms of a tetanus infection are trismus (“lockjaw”) and risus sardonicus (ironic smile of tetanus). The causative agent of tetanus is the gram-positive, spore-forming rod Clostridium tetani.
C. tetani release the exotoxin tetanospasmin, which enters the circulation and is endocytosed by neurons at the neuromuscular junction. Following retrograde transport up the axon and intercellular transfer, tetanospasmin inhibits?

Answer 20

glycine release by ultimately impairing the ability of inhibitory Renshaw neurons to release the neurotransmitters glycine and GABA. The symptoms of tetanus are due to excessive stimulation of muscles due to the removal of the inhibitory effect of glycine and GABA

Question 21

Lipopolysaccharide is a component of the outer bacterial membrane and induces a variety of immunogenic responses, including fever and cytokine release, which can lead to shock.
Degradation of host cell membranes can occur when the alpha toxin of Staphylococcus aureus and streptolysin O toxin of Streptococcus pyogenes lyse host cell membranes.
Inhibition of elongation factor 2 (EF-2) in host cells describes?

Answer 21

the exotoxin from Corynebacterium diphtheriae, the cause of diphtheria. Patients with diphtheria classically present with a gray-white pseudomembrane in the oropharynx, cervical lymphadenopathy (“bull’s neck”), and a low-grade fever.
Nonspecific activation of host T-lymphocytes describes the activity of superantigens, which lead to nonspecific, massive T-lymphocyte activation and toxic shock syndrome. Toxic shock syndrome is classically (but by no means exclusively) associated with foreign bodies (eg, tampons) and manifests with fever, hypotension, nausea and vomiting, and a diffuse erythematous rash.

Question 22

Tetanospasmin, the exotoxin released by Clostridium tetani, is endocytosed by neurons, undergoes retrograde transportation, and impairs ?

Answer 22

glycine and GABA release from inhibitory Renshaw neurons. Tetanus manifests with anorexia, risus sardonicus (ironic smile), and opisthotonos (arched back) due to excessive stimulation. In developing countries, neonatal tetanus is commonly due to umbilical stump infection.

Question 23

he patient presents with an abscess that is draining pus. Microscopic examination shows yellowish granules. The characteristic “sulfur granules,” which can be seen under the microscope and may drain through sinus tracts in skin, are indicative of an infection caused by Actinomyces israelii. A stained sample from the abscess shows branching, gram-positive rods, which further confirm Actinomyces israelii as the culprit.

Actinomyces israelii is a microaerophilic organism that can cause oral and facial abscesses. It is part of the normal flora of the mouth and gastrointestinal tract of humans, and infection is initiated usually by trauma. Treatment for this infection is ?

Answer 23

penicillin.

As the bacteria is gram positive, metronidazole provides no benefit.
Nystatin is only for fungal infections, and oxacillin is mainly used for methicillin-sensitive S. aureus.

While Nocardia spp. are weakly acid-fast and can be confused with Actinomyces, they are susceptible to sulfonamides, not penicillin.

Question 24

The classic presentation of an infection with the gram positive Actinomyces bacteria are mandibular pain after a dental procedure and draining pus that reveals sulfur (“yellowish”) granules when viewed through microscope. Treatment for Acintomyces includes? .

Answer 24

penicillin

Question 25

This patient presents with a high fever, chest pain, tachypnea, and chills. Together with findings of lobar consolidation, these signs and symptoms point to a likely diagnosis of community-acquired pneumonia (CAP). Older individuals are particularly susceptible to this disease and are most frequently infected with Streptococcus pneumoniae. Infection with these bacteria can be confirmed by a positive Quellung test result, which uses an anti-capsular antibody to elicit a “swelling” or “halo” surrounding the organism on microscopy. (This result also rules out a diagnosis of tuberculosis.) No other pathogen that causes CAP will produce this result.

A macrolide or doxycycline is the treatment of choice for the outpatient management of CAP. Macrolides act by ?

Answer 25

binding to the 23S ribosomal RNA of the 50S ribosomal subunit to inhibit protein synthesis. Doxycline acts by binding to the 30S ribosomal subunit and preventing attachment of aminoacyl-tRNA.

Question 26

Inhibition of formation of initiation complex causing misreading of messenger RNA describes the mechanism of action of aminoglycosides, which are more frequently used for gram-negative infections and not indicated in the treatment of CAP.

Blockage of tetrahydrofolic acid production refers to the combined work of trimethoprim and sulfamethoxazole, a combination used for treatment of ?

Answer 26

urinary tract and Shigella infections and for Pneumocystis jirovecii pneumonia prophylaxis.

Finally, inhibition of DNA-dependent RNA polymerase and inhibition of mycolic acid synthesis refer to the antituberculosis drugs rifampin and isoniazid, respectively; these agents are not indicated for CAP.

Question 27

Streptococcus pneumoniae is the most common cause of lobar pneumonia. Of the organisms that can cause this disease, S. pneumoniae is the only pathogen that will demonstrate capsular swelling on a Quellung test. A macrolide or doxycycline is the drug of choice for outpatient management of CAP. A macrolide acts by ?

Answer 27

binding to the 23S ribosomal RNA of the 50S ribosomal subunit to inhibit protein synthesis.

Question 28

This young pregnant woman living in Thailand had prodromal symptoms of nausea, vomiting, and watery diarrhea, in addition to liver involvement and rapid fatality. These signs and symptoms are indicative of acute hepatitis E infection, which is associated with fulminant hepatitis in pregnant women and is fatal. She might have had fulminant hepatitis B disease as well, though it is not as closely associated with pregnancy, and most persons in southern Asia are infected earlier in life. Either would be associated with?

Answer 28

hepatic necrosis.
Acute hepatitis E infection consists of two phases: a prodromal phase and an icteric phase. In the prodromal phase, typical symptoms include fever, myalgias, nausea, and vomiting. The icteric phase manifests with symptoms of cholestasis, such as jaundice, dark urine, pale stools, and pruritus. This patient’s symptoms of nausea, vomiting, and watery diarrhea were consistent with the prodromal phase. Hepatitis E is transmitted via the fecal-oral route, and infection rates are high in South and East Asia. Acute hepatitis infections may be associated with focal hepatocyte damage, which results in patchy necrosis of liver cells.

Question 29

On histologic examination of fulminant hepatitis, evidence of massive necrosis of hepatocytes in all zones of the liver (possible periportal sparing) with reticulin collapse can be identified. Healthy liver histology would be described as?

Answer 29

lobules surrounded by six portal tracts and centered on the centrilobular vein. One would be able to identify the portal triad (hepatic artery, bile duct, and hepatic portal vein) surrounded by networks of hepatocytes and bile canaliculi.

Question 30

Hepatocyte congestion is suggestive of right-sided heart failure.
Dysplastic hepatocytes with enlarged nucleolus is suggestive of primary liver cancer hepatocellular carcinoma.
Micronodular cirrhosis is associated with chronic liver disease and is a result of disorders such as alcoholism, Wilson disease, and hemochromatosis.
Well-circumscribed granuloma suggests an infection with?

Answer 30

fungal or mycobacterial pathogens, or possibly an autoimmune hepatitis.
Hepatitis E can be fatal, particularly to pregnant women, due to cirrhosis, which is seen as patchy necrosis on biopsy.