A 32-Year-Old Man Is Seen for Recurring Nausea and Vomiting

A 32-year-old man is seen for recurring nausea and vomiting. After meals, the patient experiences nausea and upper abdominal discomfort and feels full even after eating a small amount of food. He has also had several episodes of vomiting of undigested food particles, but no fever or diarrhea. The patient has a family history of type 1 diabetes and diabetic neuropathy. The oropharynx is clear on physical examination. There is no organomegaly and the abdomen is soft, nondistended, and nontender. Bowel sounds are not abnormal. Occult blood testing in the stool is negative. Endoscopy of the upper gastrointestinal tract reveals no abnormalities. Treatment with an oral medication is initiated, but it is stopped after the patient experiences abnormal involuntary movements.  A macrolide antibiotic is prescribed, which improves his symptoms.  Which of following mechanisms is most likely responsible for the improvement in this patient's symptoms?

A) Agonistic activity on motilin receptors
B) Blockade of gastrointestinal muscarinic receptors
C) Increase in intestinal fluid secretion
D) Protective effect on gastric mucosa
E) Suppression of intestinal bacterial growth