0% 0% Micro USMLE 8-30(22) (qmax 8/30 incorrect qs)

Question 1

This patient presents with lower abdominal cramping, fever and watery non-bloody diarrhea several days after being treated in the hospital for cellulitis with clindamycin. He most likely has pseudomembranous colitis, which is found primarily in hospitalized patients and caused by ?

Answer 1

Clostridium difficile. This organism causes diarrhea (usually watery, but small amounts of blood may be present), abdominal cramping (generalized or lower abdomen), and fever through the production of two exotoxins: toxin A and toxin B. This infection usually manifests after treatment with broad-spectrum antibiotics for several days such as clindamycin, ampicillin and fluoroquinolones.

Question 2

Oral vancomycin can be used for 10 to 14 days to treat the type of infection seen in this patient. Vancomycin inhibits cell wall peptidoglycan formation by?

Answer 2

binding D-ala D-ala. Metronidazole works by forming cytotoxic intermediates that damage bacterial DNA. Ampicillin works by inhibiting transpeptidase cross-linking and is associated with causing, not treating CDI. Tigecycline and fidaxomicin work by inhibiting protein synthesis by inhibiting the 30S subunit and RNA polymerase, respectively. Although oral vancomycin, tigecycline and fidaxomicin are options in the treatment of CDI, they are not first-line therapy.

Question 3

Clostridium difficile pseudomembranous colitis can manifest following treatment with broad-spectrum antibiotics, particularly with ampicillin and clindamycin. The first-line initial therapy recommended by treatment guidelines for this infection is vancomycin, which inhibits?

Answer 3

cell wall peptidoglycan formation by binding D-ala D-ala.

Question 4

An epidemic of a diarrhea has broken out in a city hospital. Colonoscopy of one of the affected patients reveals colonic inflammation with exudates and necrosis of the mucosal surface. Assays for toxin A and toxin B are positive.

Which of the following is the microbiology laboratory likely to isolate from the affected patients?

Answer 4

A gram-positive anaerobe

The description of colonic inflammation with exudates and necrosis of the mucosal surface describes the pseudomembranous colitis of Clostridium difficile, of which there have been several outbreaks. C. difficile is a gram-positive anaerobe spore-former that produces toxin A (which causes diarrhea) and toxin B (which is cytotoxic). AB toxin causes cell apoptosis. Later stages of C. difficile infection can result in necrosis as described in the vignette. Strains that produce an increased amount of these toxins have led to increased morbidity and even death associated with C. difficile colitis.

Question 5

Clostridium difficile is a?

Answer 5

gram-positive anaerobic organism that produces toxin A and toxin B, causing diarrhea and necrosis of the colonic mucosa, respectively. It is a source of increasing morbidity, and appropriate preventive measures, most notably hand washing and judicious use of antibiotics, must be employed.

Question 6

This HIV-positive patient with weakness of right upper and lower limbs (hemiparesis), disorganized meaningless speech (aphasia), ataxia (loss of muscle control), seizures, and increased signal intensity in the temporal lobe on MRI most likely has temporal lobe encephalitis.

Herpes simplex virus type 1 (HSV-1) is a ?

Answer 6

double-stranded DNA virus that can cause temporal lobe encephalitis in immunocompromised patients. Following the primary infection, the HSV-1 virus may enter a latent period, lying dormant in the trigeminal ganglion. During the latent phase, viral genetic material is preserved as an episome: circular, double-stranded DNA that is not yet integrated into the host chromosome and therefore can be detected in infected neurons.

Question 7

Initial infection with HSV-1 occurs in the skin and mucous membranes in the oropharynx. During this initial stage, the virus actively replicates and produces vesicular skin lesions that contain numerous viral particles. Subsequently, the virus may be reactivated by various stimuli including stress, menstruation, and exposure to ultraviolet light. On reactivation, viral proteins are synthesized and a reactivated infection can occur.

DNA is copied by viral DNA polymerase during lytic, not latent, replication.
Parvovirus B19 is a ?

Answer 7

single-stranded DNA virus that manifests in adults with prodromal symptoms (fever, malaise, headache) followed by pure RBC aplasia and/or symmetric arthritis of the hands, wrists, and knees. This patient’s symptoms are not consistent with parvovirus B19 infection.
Viral nucleocapsids are not produced during the latent period of virus replication.
Reverse transcriptase is not associated with this patient’s condition and therefore would not be detected in infected neurons.

Question 8

During the latent phase, HSV-1 viral genetic material is preserved as ?

Answer 8

an episome: circular, double-stranded DNA that is not yet integrated into the host chromosome and therefore can be detected in infected neurons.

Question 9

This patient with pearly white/skin-colored papules with central umbilication has the classic dermatological finding associated with molluscum contagiosum. Caused by?

Answer 9

a poxvirus, an enveloped virus with double-stranded, linear DNA, molluscum contagiosum virus is most frequently seen in children. In this population, the virus is often transmitted through skin-to-skin contact or indirect contact with fomites (e.g. gymnasium equipment). Lesions commonly occur on the chest, arms, trunk, legs, face, and intertriginous areas.
Molluscum contagiosum also occurs in adults and may be transmitted sexually. In such cases, if the patient is immunocompetent, lesions tend to be located around the genitalia and surrounding areas, as seen in this patient. Widespread, persistent, and atypical presentations may be seen in immunocompromised patients. The infection often resolves spontaneously, but may require topical therapy or surgery in immunocompromised patients.

Question 10

The organisms named in the alternate answer choices do not produce the pearly white or skin-colored papules with central umbilication seen in this patient’s skin. Among the non-enveloped RNA viruses, coxsackie virus, which causes hand, foot, and mouth disease, is most closely associated with a rash; however, this is described as red, blister-like lesions. Microsporum, a mold, (and several similar organisms) cause cutaneous infections with a pruritic, flat rash on several parts of the body. Malassezia species, al-shaped, budding yeasts, are responsible for tinea versicolor, an infection in which patches of skin become discolored due to melanocyte damage by the infecting fungus. Rickettsia are?

Answer 10

gram-negative, obligate intracellular bacteria capable of causing a rash, which is erythematous; other similar organisms also cause rashes different from those in the illustration. Infections caused by this pathogen often present with systemic symptoms, as well. Dermatological findings of Treponema pallidum, a spiral-shaped bacterium, include maculopapular rashes, characterized by flat, coarse red areas of skin; other spirochetes also cause rashes.

Question 11

This patient presents with bloody diarrhea after eating undercooked ground beef. Given her fever, tachycardia, petechiae (a sign of thrombocytopenia), elevated creatinine (which is concerning for renal failure), elevated WBC count, and decreased hemoglobin (which is concerning for anemia), without coagulopathy, this child is likely suffering from hemolytic-uremic syndrome (HUS).
HUS is most commonly associated with infection with Shigella dysentariae and Escherichia coli O157:H7– both express shiga-toxin.

Composed of two components, A and B, these toxins can cause renal failure, microangiopathic hemolytic anemia, and disseminated intravascular coagulation, as well as thrombocytopenia and fever. The B component binds to the cell membrane and allows entry of the A toxin. This component?

Answer 11

inactivates ribosomes, thereby stopping protein synthesis, which leads to cell death

Question 12

Since these pathogens are commonly acquired via the digestive tract, the cells exposed die and slough off, leading to the bloody diarrhea experienced by the patient. Both Shigella and Escherichia coli O157:H7 attach to?

Answer 12

intestinal epithelium, and the toxin (not the bacteria) invades the cell. The two pathogens can be differentiated on stool cultures.

Clinical features of HUS include:

thrombocytopenia
microangiopathic hemolytic anemia
acute renal failure

Question 13

The toxin produced by Bordetella pertussis (B. pertussis) over-activates adenylate cyclase by disabling the G-inhibitory protein. Infection with B. pertussis is not characterized by the bloody diarrhea, fever, tachycardia, and petechiae seen in this patient. The toxin produced by Corynebacterium diphtheriae (C. diphtheriae) inactivates elongation factor (EF)-2 by adenosine diphosphate ribosylation, but EF-2 is not a G protein. Infection with C. diphtheriae is not characterized by the signs and symptoms seen in this patient.

The venom produced by Latrodectus (the black widow spider), drastically increases the amount of ?

Answer 13

acetylcholine released, which causes muscle contraction until all of the acetylcholine has been used; paralysis can then occur as a result. There is no history of spider bite – which in the case of the black widow is usually immediately painful – in this patient. Staphylococcus aureus produces an exotoxin that binds major histocompatibility complex II and T-lymphocyte receptor, causing the massive release of cytokines, resulting in toxic shock syndrome.

Question 14

E. coli O157:H7 (EHEC) and Shigella are two foodborne pathogens that can lead to hemolytic-uremic syndrome (HUS). These bacteria secrete protein toxins that enter intestinal cells and inhibit protein synthesis by?

Answer 14

binding ribosomes. Some of the more common manifestations are bloody diarrhea, acute renal failure, and thrombocytopenia.

Question 15

A “barking” cough, hoarse voice, inspiratory stridor, and a positive “steeple sign” on a chest x-ray are found in patients with croup. Croup is caused by the parainfluenza virus, which is ?.

Answer 15

a single-stranded, negative-sense, linear, nonsegmented RNA virus

Question 16

This patient has systemic symptoms of fever, night sweats, and weight loss, which are likely associated with an infection caused by ?

Answer 16

her severe immunosuppression with a CD4+ count of less than 50 cells/mm³. Disseminated Mycobacterium avium–intracellulare can manifest as a severe systemic illness in immunocompromised individuals when the CD4+ count is less than 50 cells/mm³. Prophylaxis with azithromycin is at 50 cells/mm³.

Question 17

Herpes simplex virus infection can occur when the CD4+ count is less than 400/mm³ or earlier. Infection typically presents with oral or genital ulcers.
Herpes zoster infection can manifest when the CD4+ count is less than 400/mm³ or earlier. It is characterized by a painful collection of vesicles in a dermatomal pattern.
Oral thrush can occur when the CD4+ count is less than 500/mm³. It manifests with white patches and plaques on the oral mucosa.
Pneumocystis pneumonia is a common opportunistic infection when the CD4+ is less than 200 cells/mm³. It commonly manifests with?

Answer 17

fever, malaise, dyspnea on exertion, hypoxia, and a nonproductive cough.
A Toxoplasma brain abscess may occur when a patient’s CD4+ count is less than 100 cells/mm³. It most commonly manifests with focal neurologic deficits, and a CT scan will show multiple ring-enhancing lesions.

Question 18

Immunosuppression from HIV infection leads to susceptibility to various opportunistic infections, depending on a patient’s CD4+ count. CD4+ counts less than 50/mm³ predispose HIV-positive patients to ?

Answer 18

Mycobacterium avium–intracellulare (MAC) infections, which can be identified by symptoms resembling those of tuberculosis—fever, night sweats, and weight loss. Prophylactic azithromycin is administered to patients to prevent MAC infections.

Question 19

This patient’s signs and symptoms, bloody diarrhea, abdominal pain, and a fever of one day’s duration, in the context of no other recent illness, should raise suspicion for food-borne infectious gastroenteritis. Bloody diarrhea can have a number of bacterial causes, including enterohemorrhagic Escherichia coli (O157:H7), Salmonella, Shigella, Campylobacter, and Yersinia enterocolitica. In cases of gastroenteritis caused by ?

Answer 19

Salmonella infection, Salmonella enterica enteritidis is now the most commonly implicated serovar in cases of food poisoning.

Question 20

Both Salmonella and Shigella species are gram-negative rods that do not ferment lactose and are oxidase-negative. In culture, Salmonella are flagellated and motile; whereas Shigella propel themselves solely intracellularly via actin polymerization and are non-motile in culture. Non-lactose-fermenting bacteria appear clear or white on MacConkey agar.

Hektoen enteric agar is useful for ?

Answer 20

differentiating between Salmonella and Shigella because only Salmonella produces hydrogen sulfide. hydrogen sulfide forms a black precipitate when it comes into contact with thiosulfate in Hektoen agar. Shigella does not produce hydrogen sulfide, and therefore does not create a precipitate on Hektoen agar. Thus the colonies remain green without a black center.

Question 21

Campylobacter jejuni grows on Skirrow media, not MacConkey agar. Escherichia coli is a lactose fermenter and would grow pink colonies on MacConkey agar. Shigella dysenteriae does not produce sulfur and would grow clear colonies on Hektoen agar. Yersinia enterocolitica grows best at ?

Answer 21

colder temperatures and would not be motile at 37°C.

Common causes of bloody diarrhea include Salmonella, Shigella, E. coli, Campylobacter, and Yersinia. These pathogens can be differentiated by lab and culture results. Salmonella is the only producer of hydrogen sulfide, which can be detected on Hektoen agar culture.

Question 22

The organism is Giardia lamblia. Giardia can cause a long-term, smoldering infection of the small intestine resulting in few or no symptoms. The image shows the trophozoite form of the organism, which proliferates in the intestine. Symptoms occur due to?

Answer 22

loss of brush border enzymes and progressive blunting of villi. Lactase is one of the brush border enzymes that is lost with Giardia infection and can cause secondary lactose intolerance.

Question 23

This patient has a fever and a pruritic, vesicular rash with macules, papules, and pustules in different stages of healing. This is a classic case of chickenpox, which is caused by varicella-zoster virus (VZV). VZV is a highly contagious herpesvirus that is transmitted by respiratory droplets or direct contact and produces a self-limited infection. Following an infection, reactivation of latent VZV in the dorsal root ganglia can cause herpes zoster (shingles). Shingles is a sporadic disease characterized by a?

Answer 23

unilateral vesicular eruption within a dermatome, and is often associated with severe pain. Patients with herpes zoster can transmit their infection to seronegative individuals, who will then develop chickenpox.

Question 24

Autoimmune hemolytic anemia can be associated with conditions that may also cause rashes, but not the vesicular, pruritic rash seen in this patient.
Congenital deafness is more often attributed to maternal rubella or cytomegalovirus infections. Although rubella causes a widespread rash, it is not pruritic, and it would cause congenital deafness in a child only if seen in the mother during pregnancy.
Orchitis is more commonly associated with?

Answer 24

mumps infection, which manifests with inflammation of one or both parotid glands.
Subacute sclerosing panencephalitis is a complication of measles infection, which causes a widespread rash, but that is not pruritic.

Question 25

Varicella-zoster virus (VZV) is in the herpesvirus family and is responsible for causing chickenpox and herpes zoster, or shingles. Chickenpox is transmitted by respiratory droplets and produces a self-limited infection that causes a pruritic, vesicular rash. Herpes zoster is caused by reactivation of latent VZV in the dorsal root ganglia and causes a ?

Answer 25

painful unilateral vesicular eruption within a dermatome.

Question 26

Based on this patient’s history of HIV without proper medical management and presentation of purple macules and papules, he most likely has Kaposi sarcoma (KS). KS is caused by the double-stranded DNA human herpesvirus (HHV) 8, and is considered an AIDS-defining illness.

Of the answer choices, only ?

Answer 26

roseola infantum is caused by another member of the human herpesvirus family, HHV-6. Other members of the herpesvirus family are: herpes simplex virus-1 and herpes simplex virus-2 (oral and genital lesions), Epstein-Barr virus (mononucleosis and Burkitt lymphoma), cytomegalovirus (mononucleosis), and varicella-zoster virus (chickenpox and shingles).

Question 27

Hepatitis C belongs to the flavivirus family.
Hepatitis B belongs to the hepadnavirus family.
Molluscum contagiosum is caused by poxvirus which is part of the poxviridae family.
Cervical cancer is most likely caused by?

Answer 27

human papillomavirus (HPV) from the papillomavirus family

Question 28

Kaposi sarcoma is an AIDS-defining illness caused by HHV-8, which belongs to the herpesvirus family. Other members of the herpesvirus family are: ?

Answer 28

herpes simplex virus-1 and herpes simplex virus-2 (oral and genital lesions), Epstein-Barr virus (mononucleosis and Burkitt lymphoma), cytomegalovirus (mononucleosis), roseola infantum (HHV-6), and varicella-zoster virus (chickenpox and shingles).

Question 29

The patient’s weight loss and drug use are consistent with human immunodeficiency virus (HIV) infection. The oral and esophageal lesions described are thrush in an HIV-positive patient, caused by?

Answer 29

Candida albicans, a germ tube-positive fungus. Germ tubes are the initial form of hyphae that extend from the fungal spore. A standard test used to detect Candida involves incubating the specimen in a serum for 3 hours at 37°C (98.6°F) and then examining the spores for signs of germ tubes. The micrograph shows germ tubes.

candida albicans is a germ tube-positive fungus that causes white plaques (thrush) in immunocompromised individuals.

Question 30

Squamous cell carcinoma (shown in this image) is typically found in older black men with a history of smoking or lye ingestion.

Adenocarcinoma of the esophagus is typically present in patients with a long history of gastroesophageal reflux disease and Barrett esophagus.

Germ tube-negative fungi include histoplasmosis, blastomycosis (shown in the image), and coccidioidomycosis, none of which presents as?

Answer 30

oral thrush.
Herpes simplex virus esophagitis is characterized by well-circumscribed, erythematous, punched-out lesions. The image shows the cytologic changes of herpes esophagitis.