USMLE Step 1 Practice Test 13

A 4-year-old girl is brought to the emergency department because of decreased urination and pallor for 2 days. One week ago, after visiting a petting zoo, she had abdominal cramps and bloody diarrhea without fever, which have resolved. Pulse is 128/min, and blood pressure is 118/76 mm Hg. Examination shows pallor and scattered petechiae on the legs. Hemoglobin concentration is 7.8 g/dL, platelet count is 42,000/mm³, and serum creatinine concentration is 2.4 mg/dL. A blood smear shows schistocytes. A stool culture grows a gram-negative rod that does not ferment sorbitol. Which of the following is the most likely mechanism of action of the toxin responsible for this patient's condition?