Question 1

A 4-year-old boy has a yearlong history of repeated staphylococcal and streptococcal infections. He was referred to the genetic counseling unit where he was found to have depressed superoxide dismutase activity. Treatment with which of the following is most likely to be successful?

Accepted Answer

Repeated infections with depressed superoxide activity characterize chronic granulomatous disease. Interferon-$\gamma$ has been successful in treatment. Loss of adenine deaminase is the lesion found in SCID patients. $TGF-\beta$ inhibits T-cell and B-cell proliferation.

Question 2

A 23-year-old who just returned from a 2-week missionary trip to rural Haiti presents with a 2-day history of watery diarrhea. What is most likely causing his problem?

Accepted Answer

No mention of high fever or vomiting is included in the case, so the disease is most likely "traveler's diarrhea," commonly caused by enterotoxic E. coli. ETEC produces two exotoxins (LT and ST), both acting on the intestine, so they can also be called enterotoxins. It is common that when bacteria invade the gastrointestinal tract tissue, inflammation and prostaglandins are triggered, causing higher fever, abdominal pain, and diarrhea. Choice D is type I Shigella dysenteriae, which is both invasive and produces the Shiga toxin.

Question 3

A 43-year-old woman develops cervical carcinoma. What viral protein played a role in the development of the carcinoma?

Accepted Answer

Cervical carcinoma is caused by oncogenic strains of the human papillomavirus (most commonly 16, 18, and 31). The early protein E6 is associated with the oncogenic potential of human papillomavirus.

Question 4

A woman who is an intravenous drug abuser and who has been a prostitute for the past 20 years now has an aortitis. She has no mucosal ulcerations or exanthems. Her VDRL (Venereal Disease Research Laboratory) test is negative and her fluorescent treponemal antibody absorption test is positive. What is the most likely diagnosis?

Accepted Answer

The FTA-abs test detects specific antitreponemal antibody. The FTA-abs remains positive for life with or without antibiotic therapy. The VDRL test detects less specific reaginic antibodies, which decline with successful treatment, but also sometimes decline without treatment in tertiary syphilis. Therefore, based on the serologic data, this patient could have very early primary syphilis or untreated tertiary syphilis. The symptoms are consistent with tertiary syphilis.

Question 5

A strain of Influenza A virus is resistant to treatment with oseltamivir. The most likely mechanism for this resistance is

Accepted Answer

The most common mechanism of resistance to antivirals is a genetic mutation in the viral gene coding for the viral protein that interacts with the antiviral so that the antiviral no longer interacts and is effective.

Question 6

It is August. A 30-year-old man appears at your office complaining of difficulty in breathing, headache, and joint and lower back pain. He has a 101°F fever and a prominent cough. During your history taking, only one unusual activity, the thorough cleaning of an old hunting shack, was noted. The cleaning involved removal of old food containers, pots, and bedding, plus some rodent nests. He also deployed new mouse traps. Based on this history, what agent is most likely causing his symptoms?

Accepted Answer

Although several of the organisms could result in these clinical signs and symptoms, the interactions of the patient in an old hunting shack containing rodent feces point toward the Hantavirus, Sin Nombre, as the cause.

Question 7

A 10-month-old child presents with a temperature of 39.8°C (103.6°F) and lethargy. Brudzinski's and Kernig's signs are both present. Gram stain of sediment from CSF showed Gram-negative rods. What part of the routine health care could have prevented this disease?

Accepted Answer

Based on the description of the disease, the child has bacterial meningitis. The Gram stain suggests Haemophilus influenzae is the causative agent, which is confirmed by latex particle agglutination. The vaccine is a conjugate polysaccharide-protein vaccine as described in choice C. The other vaccines described belong to poliovirus (A), the adult Strep. pneumoniae vaccine (B), the infant Strep. pneumoniae vaccine (D), and the Neisseria meningitis vaccine.

Question 8

A neonate with very low Apgar scores dies 2 hours after birth. Autopsy reveals disseminated granulomatous lesions throughout; some are caseating, but they are not calcified. During her pregnancy, the mother most likely had a septicemia caused by

Accepted Answer

Listeria may cause in utero infections or may infect the baby during delivery. In utero Listeria infections are generally severe and are characterized by caseating, granulomatous lesions. Except for Toxoplasma and parvovirus, the other organisms listed cause infections acquired during birth. Toxoplasma usually manifests with calcified central nervous system lesions in the baby. Parvovirus B19 can cause anemia and hydrops fetalis when it crosses the placenta.

Question 9

A nonnucleoside analog that inhibits herpesvirus DNA replication is

Accepted Answer

Foscarnet inhibits herpesvirus DNA polymerase directly.

Question 10

The v-src oncogene of Rous sarcoma virus is a

Accepted Answer

The v-src oncogene associated with the Rous sarcoma virus codes for a tyrosine protein kinase with a biologic activity that results in cellular transformation.

Question 11

In which of the following phases of growth is a Gram-positive bacterium most susceptible to the action of penicillin?

Accepted Answer

Gram-positive bacteria would be most susceptible to penicillin in the exponential phase, because this is the phase in which cell-wall synthesis is greatest.

Question 12

The genetic mechanism responsible for the conversion of a nontoxigenic strain of Corynebacterium diphtheriae to a toxigenic strain is

Accepted Answer

A toxigenic strain of Corynebacterium diphtheriae is produced as a result of lysogenic phage conversion after a temperate bacteriophage infects a nontoxigenic strain of the organism.

Question 13

Latent infection of neurons occurs with

Accepted Answer

Herpes simplex viruses have the ability to become latent in neurons and reactivate (replicate) under certain conditions that are not well understood.

Question 14

What respiratory infection is known to increase susceptibility to pneumonia caused by Streptococcus pneumoniae?

Accepted Answer

Pneumococcal pneumonia is most frequent in patients with some damage to mucociliary elevators in the upper respiratory tracts. Antecedent measles, influenza virus infections, and alcoholism predispose patients to pneumococcal pneumonia.

Question 15

A patient with sickle cell anemia is most likely to have repeated septicemias and possible osteomyelitis with which agent?

Accepted Answer

Sickle cell anemia patients have problems with septicemias with encapsulated organisms, such as pneumococcus and Klebsiella. Of those listed, Salmonella enteritidis has a prominent capsule and it is noted for causing repeated infections in sickle cell carriers. Staphylococcus aureus (often unencapsulated but sometimes with a microcapsule) is also a cause of osteomyelitis but is much less common as a causative agent of osteomyelitis in SCD than Salmonella.

Question 16

The mother of a 9-year-old who is in the same school as another child just diagnosed with meningitis (even though the child did not know nor had any direct contact with the infected child) called the pediatrician, demanding her son be prophylactically treated. The physician suggests vaccinating the boy, doing surveillance cultures, and watching the child for symptoms. The medium that the lab will use has antibiotics in it and therefore is called a(n)

Accepted Answer

A selective medium permits growth in the presence of agents that inhibit other bacteria, in this case the normal oropharyngeal flora. A minimal medium contains the minimum quantity and number of nutrients capable of sustaining growth of the organism. A differential medium differentiates among organisms on the basis of color due to different fermentation or pH. Although you are inhibiting some bacteria, this name is not used as you are setting conditions to selectively grow the Neisseria.

Question 17

What characteristic of the influenza A virus allows genetic reassortment?

Accepted Answer

Influenza A virus, which causes a localized respiratory infection, has a segmented genome composed of eight pieces of negative-sense, single-stranded RNA, which can "reassort" when two different strains infect the same cell.

Question 18

Joe has been HIV positive for the past 6 years. His disease has been slowly progressing. Mike, his only partner for the past 3 years, is free of the disease. A plausible explanation for Mike's lack of infection would be

Accepted Answer

The absence of the obligate coreceptor CXCR4 for HIV prevents viral entry into CD41 cells. There is no evidence that NK cells, a high CD4/CD8 ratio, or anti GP-160 antibodies is protective.

Question 19

A Brazilian who left poverty and moved to Los Angeles 10 years ago has died suddenly of heart failure. Autopsy showed cardiomyopathy due to Chagas' disease. Which vector transferred the trypanosomes?

Accepted Answer

Chagas' disease (South American trypanosomiasis) is transmitted by reduviid bugs that defecate as they bite. The trypanosome is actually in the feces, which is scratched into the bite site. Reduviid bugs are also called kissing, assassin, or cone-nosed bugs.

Question 20

An autoimmune disease characterized by absence of T cells, hypocalcemia, and tetany with lowered cell-mediated immunity is

Accepted Answer

Congenital thymic aplasia is caused by an unknown intrauterine injury to the third and fourth pharyngeal pouches around the 12th week of gestation. Failure of the parathyroid glands to develop results in tetany and hypocalcemia. Injury to the thymus accounts for the absence of T cells.

Quiz 11: Travelers Diarrhea: Identifying the Causative Agent