Question

A 45-year-old man presents with chronic diarrhea, steatorrhea, and weight loss. A D-xylose absorption test shows low serum and urinary D-xylose levels. What is the most likely cause of this finding?

• A. Ileal disease
• B. Bacterial overgrowth
• C. Pernicious anaemia
• D. Pancreatitis

Hint:

D-Xylose Test Interpretation Rule:
Normal D-Xylose = Pancreatic insufficiency (mucosa is intact).
Abnormal/Low D-Xylose = Small bowel mucosal disease (Celiac) OR Bacterial Overgrowth (SIBO).

Answer 1

The Correct Option is B

Concept:
A middle-aged man presents with a classic malabsorption syndrome characterized by chronic diarrhea, significant weight loss, and fat malabsorption (steatorrhea). A specialized D-xylose absorption test yields abnormally low levels in both the serum and the urine.
The clinical task requires correctly interpreting the physiological meaning of an abnormal D-xylose test to definitively narrow down the anatomical and pathophysiological etiology of the malabsorption.
Explanation:
• The D-xylose absorption test is a classic, specialized diagnostic tool historically used to differentiate between malabsorption caused by pancreatic exocrine insufficiency and malabsorption caused by intestinal mucosal pathology.

• D-xylose is a simple, five-carbon monosaccharide. Unlike complex carbohydrates, it does absolutely not require any digestion by pancreatic enzymes or brush-border enzymes to be absorbed.

• Upon oral ingestion, it is absorbed directly and entirely intact by the healthy, functioning mucosa of the proximal small intestine (the duodenum and jejunum). It is then rapidly filtered and excreted unchanged by the kidneys into the urine.

• In a patient with chronic pancreatitis (Option D), the small intestinal mucosa remains perfectly healthy. Therefore, the D-xylose is absorbed completely normally, yielding a normal, high level of the sugar in both the blood and urine.

• An abnormally low D-xylose test explicitly indicates that the sugar never successfully crossed the intestinal barrier into the bloodstream.

• This low result classically points to extensive small bowel mucosal disease (such as Celiac sprue, Tropical sprue, or Whipple's disease), which physically destroys the absorptive surface area.

• Crucially, Small Intestinal Bacterial Overgrowth (SIBO) (Option B) is another major, classic cause of a falsely low D-xylose absorption test.

• In SIBO, an abnormally large, pathological population of colonic bacteria colonizes the proximal small intestine. When the patient ingests the D-xylose load, these bacteria rapidly ferment and metabolize the sugar for their own energy before it can ever reach the mucosal surface to be absorbed by the host.

• As a result, the serum and urinary levels of D-xylose are low. Notably, administering a short course of broad-spectrum antibiotics will eradicate the bacteria and completely normalize the D-xylose test, confirming the diagnosis.

• Ileal disease (Option A) primarily affects B12 and bile acid absorption, leaving proximal D-xylose absorption intact.
Final Answer:
A low D-xylose absorption test indicates a failure of the sugar to reach the bloodstream, which is classically caused by either extensive mucosal disease or bacterial overgrowth metabolizing the sugar.