Review Question - QID 216665

A 28-year-old woman presents to her primary care physician due to a recent history of weight loss and malaise. She reports that she has had watery bowel movements over the past month. She denies any fevers or blood in her stools. She recently traveled to the Caribbean for 6 months as part of her work. She is a former smoker. She typically practices a vegan diet but did consume dairy on this recent trip to the Caribbean. On physical exam, her temperature is 98.6°F (37°C), blood pressure is 110/80 mmHg, pulse is 90/min, and respirations are 12/min. Her abdomen is soft and nontender. She has increased bowel sounds. Serum lab tests are performed with the following results:

Hemoglobin: 10.1 g/dL
Mean corpuscular volume: 111 µm^3
Hematocrit: 30.3%
Platelet count: 250,000/mm^3

Small bowel biopsy reveals blunting of villi and infiltration of inflammatory cells (lymphocytes, plasma cells, and eosinophils). Which of the following is the most likely cause of this patient's presentation?