Kaplan USMLE Step 3: Woman reports progressive weakness and tingling

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 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.

A 22-year-old woman comes to the emergency department complaining of weakness. For the last two weeks, she has suffered from progressive weakness and tingling. First, the symptoms seemed mainly confined to her feet and ankles. Now, she has difficulty moving her entire lower extremities. About a month ago she had a severe case of bloody diarrhea. Otherwise, she has been healthy and has no chronic medical conditions. She has no HIV risk factors and does not use any illicit substances. Physical examination is remarkable for symmetric lower-extremity weakness with absent reflexes. The patient is admitted to the hospital and over the next week requires intubation and mechanical ventilation for hypercarbic respiratory failure. The patient slowly improves and, three months later, has returned to her previous functioning. Which of the following was the most likely pathogen causing this patient’s bloody diarrhea?

A. Campylobacter jejuni

B. Escherichia coli O157:H7

C. Salmonella typhi

D. Shigella dysenteriae

E. Yersinia enterocolitica

The correct answer is A.

This patient has Guillain-Barré syndrome, a form of polyradiculoneuropathy recognized by its three cardinal features of paralysis: acute (with symptoms reaching a peak in about a month), areflexic (weakness with diminished or absent reflexes), and ascending (paralysis and dysesthesias beginning in the toes and ascending to include the respiratory muscles). This syndrome may be idiopathic but is also associated with Campylobacter jejuni and HIV infection.

Choice B. E. coli O157:H7 is a species of enterohemorrhagic E. coli that, in addition to bloody diarrhea, can cause hemolytic uremic syndrome (HUS) or thrombotic thrombocytopenic purpura (TTP) in the postinfectious stage of the illness.