You are answering a USMLE-style medical question. Question: A 30-year-old man presents to the emergency room complaining of malaise. Two hours prior to presentation, he developed muscle cramps and nausea. He has had 3 episodes of non-bloody watery stools since his symptoms started. He reports that he has experienced similar symptoms multiple times over the past 5 years since he started working at his local zoo as a reptilian caretaker. His medical history is unremarkable. He takes fish oil and a multivitamin daily. His temperature is 101°F (38.3°C), blood pressure is 130/90 mmHg, pulse is 90/min, and respirations are 17/min. On exam, he demonstrates sensitivity to bright light. He is tired-appearing but alert and fully oriented. A stool sample and culture demonstrates abundant non-lactose fermenting, oxidase-negative, and H2S-producing gram-negative bacilli. The pathogen responsible for this patient’s condition relies on a virulence factor with which of the following mechanisms of action? Options: A) ADP ribosylation of elongation factor 2 B) Blocking antibody attack on the O antigen C) Cleaving immunoglobulin A molecules D) Cleaving lecithin Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.