Abdominal Distension

A 65-year-old man with GERD, type 2 diabetes, hypertension, and hyperlipidemia presents with persistent abdominal symptoms, including bloating, discomfort, flatulence, and diarrhea despite dietary adjustments. Vital signs are normal, and examination reveals a mildly distended abdomen, normal bowel sounds, moist mucous membranes, and no leg swelling or rashes. Laboratory tests show elevated A1c (7.6%), cholesterol, triglycerides, and slightly low B12 levels. Ultrasonography reveals a mildly fatty liver. Colonoscopy and upper endoscopy show normal mucosa with mild chronic gastritis, with biopsies taken for further evaluation, and no evidence of celiac disease. Which of the following diagnoses is most likely,