If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 12-year-old boy has a productive cough characterized by large volumes of mucoid, foul-smelling sputum. He has a history of recurrent pneumonias, epigastric pain, and increased stool frequency since age two. His mother states that he was delivered via a normal vaginal delivery with meconium ileus noted at birth. His temperature is 38°C (100.4°F), pulse is 96 per minute, blood pressure is 110/82 mm Hg, and respirations are 20 per minute. Physical examination reveals height and weight in the thirty-ninth percentile, few small nasal polyps bilaterally, diffuse crackles on lung auscultation, hyperresonance with chest percussion, and scattered petechiae and purpura. Cultures of the sputum grow oxidase-positive colonies with a fruity aroma on blood agar.
Which of the following is the most likely mechanism of action of the exotoxin produced by the causal organism?
A. Activation of macrophages to produce proinflammatory cytokines.
B. Block release of acetylcholine.
C. Block release of GABA and glycine.
D. Increasing levels of cAMP.
E. Inhibition of protein synthesis.
The correct answer is E.
Kaplan Medical explains why
This boy has cystic fibrosis (CF). Patients with CF have bronchiectasis (commonly associated with copious sputum production), recurrent pneumonia, nasal polyps, pancreatic insufficiency, and steatorrhea. Epigastric pain and fat soluble vitamin K deficiency (reflected in the presence of petechiae and purpura) arise as a consequence of pancreatic insufficiency. Patients would also have a history of meconium ileus at birth and lower height and weight (mean 37 percent) due to malabsorption.
However, the question directly relates to the infectious agent causing the pneumonia: Pseudomonas aeruginosa, an oxidase-positive bacterium with a fruity aroma. Pseudomonas produces blue-green pigments that act as siderophores when grown in an iron-limited environment, but are often colorless on blood agar. P. aeruginosa is an aerobic, Gram-negative rod that produces an exotoxin (exotoxin A) that ADP-ribosylates and inhibits eukaryotic elongation factor-2 (eEF-2), which is a GTP-binding protein involved in the eukaryotic translation of proteins (Corynebacterium diphtheriae also produces an exotoxin with a similar mechanism of action to inhibit protein synthesis). P. aeruginosa is a common pathogen cultured from CF patients.
Why the other answers are wrong
Choice A: Activation of macrophages to produce proinflammatory cytokines is the mechanism by which lipopolysaccharide causes damage. This is endotoxin (an integral component of Gram-negative outer membranes) and is not to be confused with exotoxin.
Choice B: Blocking release of acetylcholine is specific for Clostridium botulinum, which produces a neurotoxin that causes a flaccid paralysis.
Choice C: Blocking release of GABA and glycine is specific for Clostridium tetani, which produces a neurotoxin that causes a spastic paralysis.
Choice D: Increasing levels of cAMP refers to the toxins of enterotoxigenic Escherichia coli, Vibrio cholerae, Bordetella pertussis, and Bacillus anthracis. These exotoxins each increase cellular fluid loss, resulting in edema or diarrhea.
Tips to remember
- Cystic fibrosis (CF) is identified by the findings of abnormal sputum production and pancreatic insufficiency.
- CF patients suffer from frequent infections with Pseudomonas aeruginosa.
- This bacterium produces exotoxin A, which inhibits eukaryotic elongation factor 2 (eEF-2), inhibiting protein synthesis.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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