A 38-year-old woman comes to the office due to abdominal bloating, excessive flatulence, and diarrhea over the past 3 months. She has 4 or 5 nonbloody, loose stools a day, including at night. The patient has a history of obesity and lost 30 kg (66 lb) after undergoing Roux-en-Y gastric bypass surgery 3 years ago. She takes multiple vitamin supplements daily but has not taken antibiotics recently. The patient has not traveled recently or been exposed to anyone with diarrheal illness. Temperature is 37 C (98.6 F), blood pressure is 130/84 mm Hg, and pulse is 78/min. BMI is 28 kg/m². Abdominal examination shows no abnormalities. Which of the following is the most likely cause of this patient's symptoms?

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11ec25c8_d90a_4ab5_bc81_a79417aa19cb_MD0003_00 This patient has small intestinal bacterial overgrowth (SIBO), presenting with chronic abdominal bloating, excessive flatulence, and nonbloody diarrhea. The proximal small intestine normally contains minimal bacterial colonization due to gastric acidity and peristalsis; however, Roux-en-Y gastric bypass procedures create a blind loop of intestine that allows for excessive bacterial growth. Conditions that alter intestinal motility (eg, systemic sclerosis, diabetes mellitus), anatomy (eg, strictures), or gastric/pancreatic secretions (eg, atrophic gastritis, chronic pancreatitis) also predispose to SIBO. Patients typically develop bloating, flatulence, abdominal discomfort, and watery diarrhea; in severe cases, malabsorption and weight loss are seen. Laboratory studies are frequently normal but may demonstrate nutritional deficiencies (eg, vitamin B₁₂, fat-soluble vitamins) and macrocytic anemia. In contrast, folic acid and sometimes vitamin K may be elevated due to increased production from enteric bacteria. Diagnostic tests include carbohydrate breath tests (eg, lactulose, glucose) that measure the production of hydrogen and/or methane by intestinal flora. Endoscopy with jejunal aspirate and culture showing increased bacterial burden (>10³colony-forming units/mL) is the gold standard but invasive. Management involves empiric antibiotics (eg, rifaximin). ³ (Choice B) Clostridioides difficile infection causes watery diarrhea, but symptoms are typically acute, and patients with prolonged symptoms commonly show signs of volume depletion (eg, hypotension) due to heavy fluid losses. C difficile infection is uncommon without recent hospitalization or antibiotic use. (Choice C) Dumping syndrome is a common complication of gastric bypass surgery. It is caused by rapid emptying of high-carbohydrate foods into the small bowel, which causes osmotic fluid shifts from the plasma to the intestinal lumen, leading to sympathetic activation, tachycardia, diaphoresis, and flushing. Symptoms occur within 30 minutes of meals, and nocturnal symptoms are not common. (Choice D) Irritable bowel syndrome is characterized by chronic abdominal pain, altered bowel habits, and symptom relief after a bowel movement. Nocturnal symptoms are generally not present. (Choice E) Stomal obstruction, also called anastomotic stricture, is a narrowing of the lumen at the anastomotic site. It typically occurs in the first 1-2 months after gastric bypass surgery, and although it can cause bloating, it typically presents with dysphagia and vomiting of undigested food rather than with diarrhea. Educational objective: Small intestinal bacterial overgrowth is characterized by bloating, excessive flatulence, and watery diarrhea; malabsorption and nutritional deficiencies may also occur. It is a common complication of Roux-en-Y gastric bypass procedures and other conditions that alter intestinal anatomy or motility. References: Review article: small intestinal bacterial overgrowth-prevalence, clinical features, current and developing diagnostic tests, and treatment. (http://www.ncbi.nlm.nih.gov/pubmed/23957651)

A 38-Year-Old Woman Comes to the Office Due to Abdominal