A 26-year-old man of Native American origin presents with a three-month history of watery diarrhea occurring 4-5 times daily.

A 26-year-old man of Native American origin presents with a three-month history of watery diarrhea occurring 4-5 times daily. Along with episodes of diarrhea, he experiences crampy abdominal pain, abdominal distension, and flatulence. He has not had any blood in his bowel movements, nor has he had any associated fevers, weight loss, nausea, or vomiting. He denies nighttime bowel movements and says that they “mostly occur after eating.” He was previously healthy. Which of the following is the most likely mechanism for his diarrhea? Deficiency in intestinal lactase (or lactose intolerance). Defective membrane transport or absorption of intestinal epithelium. Bacterial infection. Inflammation of the mucosa and submucosa of the rectum. Transmural inflammation of the bowel wall.