You are answering a USMLE-style medical question. Question: A 38-year-old woman comes to the physician because of persistent diarrhea and fatigue for 1 month. She has had up to 12 watery stools per day. Sometimes she awakens at night because she has to defecate. She has abdominal bloating and cramps. She has had a 2-kg weight loss in the past month. She returned from a trip to Costa Rica 7 weeks ago. She has a history of bronchial asthma and bulimia nervosa. She works as a nurse. She does not smoke and drinks 1–2 beers on the weekend. Current medications include fish oil, a multivitamin, and a salbutamol inhaler. Her temperature is 36.8°C (98.2°F), pulse is 65/min, and blood pressure is 100/75 mm Hg. Examination shows dry mucous membranes. Abdominal and rectal examinations are unremarkable. Laboratory studies show: Hematocrit 46% Leukocyte Count 6,500/mm3 Platelet Count 220,000/mm3 Serum Na+ 136 mEq/L K+ 3.2 mEq/L Cl- 102 mEq/L HCO3- 33 mEq/L Mg2+ 3.3 mEq/L Urea nitrogen 14 mg/dL Creatinine 0.8 mg/dL Abdominal ultrasound shows no abnormalities. Colonoscopy shows dark colored mucosa with interspersed white spots. Biopsies of colonic mucosa are obtained and sent for pathological evaluation. Which of the following is the most likely underlying cause of this patient's symptoms?" Options: A) Celiac disease B) Carcinoid tumor C) VIPoma D) Medication abuse Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.