**Question:** A d-xylose test was requested on a patient with history of long standing steatorrhea '5' hour urine sample showed <4.0 gm excretion after giving 25 gm of d-xylose. The most likely diagnosis is: A. Familial Malabsorption B. Inflammatory Bowel Disease C. Celiac Disease D. Intestinal Malabsorption due to Surgery **Correct Answer:** C. Celiac Disease **Core Concept:** The d-xylose test is a diagnostic test for malabsorption syndromes, particularly those involving small bowel dysfunction. Malabsorption syndromes are conditions characterized by impaired absorption of nutrients from the small intestine, leading to deficiencies in essential vitamins, minerals, and proteins. The d-xylose test is based on the measurement of urinary d-xylose excretion after oral administration of d-xylose. Normal excretion is typically between 10-20 grams, while values below 4 grams indicate impaired d-xylose absorption. **Why the Correct Answer is Right:** In this case, the patient has long-standing steatorrhea ('fatty stools'), suggesting malabsorption. The low d-xylose excretion in the urine (less than 4 grams) indicates impaired d-xylose absorption. Among the given options, Celiac Disease (CD) is a common cause of malabsorption. **Why Each Wrong Option is Incorrect:** A. Familial Malabsorption: This is a rare genetic disorder causing malabsorption, but the patient's history of long-standing steatorrhea suggests a more specific underlying cause. B. Inflammatory Bowel Disease (IBD): Inflammatory Bowel Disease (Crohn's disease or ulcerative colitis) typically presents with diarrhea rather than steatorrhea, and intestinal permeability tests are more sensitive in diagnosing IBD. D. Intestinal Malabsorption due to Surgery: This is a possible cause of malabsorption, but the long-standing history of steatorrhea suggests a more specific underlying cause. **Clinical Pearl:** Celiac Disease (CD) is an autoimmune disorder where ingestion of gluten (a protein found in wheat, barley, and rye) triggers an immune response against enterocytes (intestinal cells) leading to villous atrophy and impaired nutrient absorption. The patient's long-standing steatorrhea suggests a more specific underlying cause than Familial Malabsorption, which is a rare genetic disorder. **Why the Correct Answer is Right:** Celiac Disease (CD) is a common cause of malabsorption due to enterocyte damage, villous atrophy, and impaired nutrient absorption. Intestinal permeability tests, such as lactulose:mannitol ratio, may be more sensitive in diagnosing CD, but a low d-xylose absorption test is a simpler, non-invasive, and readily available test for diagnosing malabsorption syndromes like CD. **Why