Question 1

A 6-month-old infant is brought to the emergency department (ED) having woken up from a nap with a weak cry, ptosis, unreactive pupils, and the inability to lift her arms. She has had routine well-baby care. There is no sign of physical abuse or rash, and she is afebrile. The physician puts the child on a respiration monitor and calls the infectious disease consultant. What is most likely causing the problem?

Accepted Answer

The symptoms are classical for botulism. Common sources of botulinum toxin include canned home goods, and, in cases of infant botulism, household dust or honey. It is most likely that this is from the ingestion of environmental spores (dust or honey) rather than from canned goods, although that is a possibility. Listeria meningitis would be very uncommon this long after birth, and the child would probably be febrile and not have the descending paralysis. She should not have Streptococcal meningitis as she is vaccinated, and it would present as a febrile disease with a stiff neck. It is also not typical for viral meningitis.

Question 2

What is the mechanism of action of the toxin?

Accepted Answer

Botulinum toxin (a metalloprotease) cleaves docking proteins and blocks exocytosis of acetylcholine from storage vesicles, producing a flaccid paralysis. Choline acetyltransferase, choice C, is an enzyme catalyzing synthesis of acetylcholine from an acetate and choline. Sodium-dependent transport of choline can be blocked by hemicholinium (choice B). Enzyme acetylcholinesterase is responsible for catalyzing hydrolysis of acetylcholine (choice D). Acetylcholine synapses at the ganglia of many neurons and tissues, and this step is not blocked by botulinum toxin (choice E).

Question 3

Five children in Mrs. Thompson's third grade class develop a disease which begins as a bright red rash on the face, turns violet after a few days, then disappears; then, a maculopapular rash appears on the trunk, buttocks, and extremities. It soon fades from the trunk but persists on the thighs and forearms. Two children have also had a slight fever and a sore throat, but none were terribly sick. The genetic material of the most likely causative infectious agent is

Accepted Answer

The disease is most probably erythema infectiosum (slapped cheek syndrome) caused by parvovirus B19, a single-stranded DNA virus.

Question 4

If one were to create a single antigen vaccine to have prevented the outbreak and for use as a vaccine in women prior to pregnancy, what viral molecule would be most likely to lead to the production of successful neutralizing antibodies?

Accepted Answer

The disease is most probably erythema infectiosum (slapped cheek syndrome) caused by parvovirus B19, a single-stranded DNA virus. Parvovirus B19 is naked, so protective neutralizing antibodies would be to the viral capsid. It does not carry a polymerase but uses cellular polymerases, which is why it requires replicating cells. It is not enveloped, so it has no glycoproteins or any matrix proteins.

Question 5

Which of the following bacterial compounds is important in colonization?

Accepted Answer

Pilin is the main protein component of the pili involved in bacterial adherence.

Question 6

What is the function of the cyclic AMP binding proteins in facultative bacteria?

Accepted Answer

Cyclic AMP binding proteins bind to specific DNA sequences near promoters, which facilitate RNA polymerase binding to the promoters, thus enhancing transcription of genes associated with those promoters.

Question 7

An 18-year-old male college student came to the Student Health Services complaining of a sore throat, fatigue, and difficulty in swallowing. A physical examination showed an enlarged spleen; a palpable, tender liver; and swollen cervical lymph nodes. His blood is positive for heterophile antibody. What would you expect to see in a blood smear?

Accepted Answer

The young man has infectious mononucleosis caused by EBV, which produces a heterophile antibody that is the basis of the Monospot test, and the Downey cells, which are reactive T cells.

Question 8

A 17-year-old girl presents to your gynecology clinic with complaints of recurring pelvic pain. A probable diagnosis of pelvic inflammatory disease is made. She is a senior at a suburban high school, is involved in orchestra, and has been on the honor roll most years. She does not do drugs. She has had an equally "squeaky clean" type of boyfriend since last fall, and 6 months ago they had unprotected sex on three occasions. What is the most likely causative agent?

Accepted Answer

Chl. trachomatis and N. gonorrhoeae are the only two choices listed commonly causing PID. However, Chl. trachomatis is about 4 times more common; in fact, it is the most common bacterial STD in the general population at large. N. gonorrhoeae is more likely to be seen in economically disadvantaged urban teens.

Question 9

A 26-year-old man presents with cellulitis two days after he is bitten by his girlfriend's cat. What is the most likely dominant organism involved in the infection?

Accepted Answer

Pasteurella multocida is a dominant organism in the cat's mouth. Choice A, Bartonella, is the causative agent of cat scratch fever and is less common in bites. Choice B, Calymmatobacterium, causes a sexually transmitted infection, and Toxoplasma (choice D) is associated with cats but transmitted by their feces, not bites. Choice E, tetanus, is not likely unless the man has not been vaccinated.

Question 10

What compound is only found in Gram-positive bacteria?

Accepted Answer

Teichoic and teichuronic acids, which are polymers containing ribitol or glycerol, are found in the cell walls or cell-wall membranes of Gram-positive bacteria.

Question 11

The parents of an 18-month-old girl appear at the emergency department of a hospital with their daughter, who had an abrupt onset of vomiting and incidences of watery diarrhea. The symptoms began 2 days previously at her day care center. She refuses to eat or drink and is very lethargic. What property of the infectious agent causing these symptoms allowed the preparation of a vaccine that could have prevented them?

Accepted Answer

Rotaviruses (which are segmented, dsRNA viruses) are the most frequent cause of diarrheas in infants. RotaTeq and Rotarix vaccines are available for prevention.

Question 12

A 22-year-old female student complains of excessive fatigue for the past 2 months. Polyarthralgia and vasculitis is noted on examination. Lab tests reveal multiple autoantibodies including antidouble-stranded DNA. The most logical diagnosis is

Accepted Answer

These signs and symptoms are characteristic of lupus erythematosus, with antibodies to double-stranded DNA being selective for SLE. Although autoantibodies are present in Hashimoto's thyroiditis, anti-dsDNA is absent.

Question 13

A 29-year-old woman with acquired immunodeficiency syndrome has severe, nonresolving watery diarrhea. It is not gray or greasy. Acid-fast oocysts are seen in the stools. What is the most likely causative agent?

Accepted Answer

Oocysts are only formed by protozoans, eliminating A, C, and E. Acid-fast oocysts are only found in Cryptosporidium, Cyclospora, or Isospora infections.

Question 14

An elderly woman presents with chronic fatigue, difficulty sleeping, a rapid heartbeat, and bulging eyes. Lab tests revealed autoantibodies to thyroid-stimulating hormone. The most likely diagnosis is

Accepted Answer

The autoantibodies to TSH in Graves' disease compete with TSH for its receptor site and mimic TSH activity. Pernicious anemia is characterized by antibodies to the gastric parietal cell and intrinsic factor, resulting in an inability to absorb vitamin $B_{12}$.

Question 15

A 50-year-old woman from Key West presents with a mosquito-borne viral disease in which her antiviral antibodies from an infection nearly a decade ago were thought to have an "enhancing" effect so that the current reinfection with the virus caused a more serious bleeding disease. What is the causative agent?

Accepted Answer

Dengue virus is an arbovirus that causes a disease sometimes known as "break bone fever" because of the muscle and joint pain associated with it. Infection by a different virus serotype following a primary infection causes a more serious disease that is thought to be due to the enhancing effect that antibodies to the initial virus have on the second virus's infection properties.

Question 16

A 22-year-old woman presents with vaginal itching and erythema as well as a discharge which is thick and white. External erythema is also present with discrete pinpoint lesions off the edge. The discharge pH is 4.7 (normal). Amine test is negative as is the genetic probe test for Neisseria. What is the most likely cause?

Accepted Answer

The discharge is characteristic of Candida overgrowth. Note that no foul odor is mentioned, and the negative amine test "rules down" bacterial vaginosis and Trichomonas. Both yeast vaginitis and yeast diaper rash have the described satellite lesions outside the area of major erythema. Gardnerella is the prominent agent in bacterial vaginosis. The normal pH range is 4.5 to 5 and yeast vaginitis is usually in that range.

Question 17

An antibiotic that has a $\beta$-lactam ring in its structure is

Accepted Answer

Cephalosporin drugs have the $\beta$-lactam ring, as do the penicillins. They also inhibit cell-wall biosynthesis and are inactivated by some $\beta$-lactamases.

Question 18

Which bacterial gene transfer process would be inhibited by free extracellular exonucleases?

Accepted Answer

In transformation, the DNA is extracellular before it is picked up by the competent cells; during this period, the DNA is subject to the extracellular exonucleases. Because the DNA in generalized and specialized transduction is protected extracellularly by the virus capsid, it is not subject to extracellular exonucleases. In conjunction, the DNA is never outside of a cell. Transposition is a mechanism of inserting a transposon into another molecule of DNA and has no extracellular transport mechanism associated with it.

Question 19

After a trip to Peru to adopt a 6-month-old baby, a 32-year-old woman and her new baby both develop profuse, watery diarrhea with flecks of mucus. Both are hospitalized because of the severity and rapidity of the dehydration, but neither one is febrile. What is the most likely causative agent?

Accepted Answer

The mention of Peru should raise suspicion of cholera. (Rotavirus would have to be ruled out for the baby, but it is not one of the choices.) The loss of fluids and electrolytes is rapid and most severe with Vibrio cholerae. The lack of fever and lack of pus and blood in the stools suggests that the causative agent is not Campylobacter, Salmonella, or Shigella. The lack of blood also decreases the chance of Escherichia coli O157. If untreated, cholera may lead to dehydration, hemoconcentration, and hypovolemic shock.

Question 20

A 19-year-old pregnant student presents at her health service allergy clinic with rhinorrhea, sneezing, itching, and conjunctivitis. A RAST test finds IgE antibodies to ragweed antigens. A characteristic of this condition is

Accepted Answer

The cytokine IL-4 is necessary for the switch to IgE synthesis. Desensitization can occur with repeated injections of allergen, not IgE. IgE antibody does not cross the placenta. Drugs that elevate cGMP enhance symptoms.

Quiz 11: Travelers Diarrhea: Identifying the Causative Agent

What is the mechanism of action of the toxin?

If one were to create a single antigen vaccine to have prevented the outbreak and for use as a vaccine in women prior to pregnancy, what viral molecule would be most likely to lead to the production of successful neutralizing antibodies?

Which of the following bacterial compounds is important in colonization?

What is the function of the cyclic AMP binding proteins in facultative bacteria?

A 26-year-old man presents with cellulitis two days after he is bitten by his girlfriend's cat. What is the most likely dominant organism involved in the infection?

What compound is only found in Gram-positive bacteria?

A 22-year-old female student complains of excessive fatigue for the past 2 months. Polyarthralgia and vasculitis is noted on examination. Lab tests reveal multiple autoantibodies including antidouble-stranded DNA. The most logical diagnosis is

A 29-year-old woman with acquired immunodeficiency syndrome has severe, nonresolving watery diarrhea. It is not gray or greasy. Acid-fast oocysts are seen in the stools. What is the most likely causative agent?

An elderly woman presents with chronic fatigue, difficulty sleeping, a rapid heartbeat, and bulging eyes. Lab tests revealed autoantibodies to thyroid-stimulating hormone. The most likely diagnosis is

An antibiotic that has a $\beta$ -lactam ring in its structure is

Which bacterial gene transfer process would be inhibited by free extracellular exonucleases?

After a trip to Peru to adopt a 6-month-old baby, a 32-year-old woman and her new baby both develop profuse, watery diarrhea with flecks of mucus. Both are hospitalized because of the severity and rapidity of the dehydration, but neither one is febrile. What is the most likely causative agent?

A 19-year-old pregnant student presents at her health service allergy clinic with rhinorrhea, sneezing, itching, and conjunctivitis. A RAST test finds IgE antibodies to ragweed antigens. A characteristic of this condition is

Quiz 11: Travelers Diarrhea: Identifying the Causative Agent

What is the mechanism of action of the toxin?

If one were to create a single antigen vaccine to have prevented the outbreak and for use as a vaccine in women prior to pregnancy, what viral molecule would be most likely to lead to the production of successful neutralizing antibodies?

Which of the following bacterial compounds is important in colonization?

What is the function of the cyclic AMP binding proteins in facultative bacteria?

A 26-year-old man presents with cellulitis two days after he is bitten by his girlfriend's cat. What is the most likely dominant organism involved in the infection?

What compound is only found in Gram-positive bacteria?

A 22-year-old female student complains of excessive fatigue for the past 2 months. Polyarthralgia and vasculitis is noted on examination. Lab tests reveal multiple autoantibodies including antidouble-stranded DNA. The most logical diagnosis is

A 29-year-old woman with acquired immunodeficiency syndrome has severe, nonresolving watery diarrhea. It is not gray or greasy. Acid-fast oocysts are seen in the stools. What is the most likely causative agent?

An elderly woman presents with chronic fatigue, difficulty sleeping, a rapid heartbeat, and bulging eyes. Lab tests revealed autoantibodies to thyroid-stimulating hormone. The most likely diagnosis is

An antibiotic that has a β\betaβ -lactam ring in its structure is

Which bacterial gene transfer process would be inhibited by free extracellular exonucleases?

After a trip to Peru to adopt a 6-month-old baby, a 32-year-old woman and her new baby both develop profuse, watery diarrhea with flecks of mucus. Both are hospitalized because of the severity and rapidity of the dehydration, but neither one is febrile. What is the most likely causative agent?

A 19-year-old pregnant student presents at her health service allergy clinic with rhinorrhea, sneezing, itching, and conjunctivitis. A RAST test finds IgE antibodies to ragweed antigens. A characteristic of this condition is

An antibiotic that has a $\beta$ -lactam ring in its structure is