Which mechanism explains the increased risk of calcium oxalate stones in short bowel syndrome?

Which mechanism explains the increased risk of calcium oxalate stones in short bowel syndrome?

• A. Decreased calcium binding in the gut
• B. Accelerated oxalate absorption in the colon
• C. Increased calcium excretion by kidneys
• D. Reduced oxalate production by gut bacteria

Answer: (B)

In short bowel syndrome, fat malabsorption leaves fatty acids in the colon. These fatty acids bind calcium to form insoluble soaps, which reduces the amount of free calcium available in the gut to bind oxalate. Normally, calcium binds oxalate in the gut and prevents its absorption; when there’s less free calcium, oxalate is absorbed more readily in the colon. The increase in absorbed oxalate raises urinary oxalate (hyperoxaluria), promoting calcium oxalate stone formation. This direct link—oxalate absorption in the colon increasing due to low available calcium from fat-bound calcium—explains the heightened stone risk best. Other options aren’t the primary mechanism: decreased calcium binding in the gut relates to the same problem but isn’t the direct driver of increased oxalate absorption; increased renal calcium excretion is a downstream effect that doesn’t explain intestinal oxalate absorption; reduced oxalate production by gut bacteria would tend to lower oxalate availability, not raise it.

In short bowel syndrome, fat malabsorption leaves fatty acids in the colon. These fatty acids bind calcium to form insoluble soaps, which reduces the amount of free calcium available in the gut to bind oxalate. Normally, calcium binds oxalate in the gut and prevents its absorption; when there’s less free calcium, oxalate is absorbed more readily in the colon. The increase in absorbed oxalate raises urinary oxalate (hyperoxaluria), promoting calcium oxalate stone formation. This direct link—oxalate absorption in the colon increasing due to low available calcium from fat-bound calcium—explains the heightened stone risk best.

Other options aren’t the primary mechanism: decreased calcium binding in the gut relates to the same problem but isn’t the direct driver of increased oxalate absorption; increased renal calcium excretion is a downstream effect that doesn’t explain intestinal oxalate absorption; reduced oxalate production by gut bacteria would tend to lower oxalate availability, not raise it.