A 40-year-old woman underwent massive small bowel resection for superior mesenteric artery thrombosis, leaving 90 cm of jejunum with loss of the terminal ileum and entire colon. Two years later she develops steatorrhoea, macrocytic anaemia, and recurrent calcium oxalate renal stones. What is the mechanism of her renal stones?
- A Excess urinary citrate binding dietary calcium
- B Unabsorbed fatty acids bind calcium, allowing free oxalate to be absorbed from the colon ✓
- C Vitamin D excess causing hypercalciuria
- D Reduced tubular secretion of oxalate due to volume depletion
Correct answer: B. Unabsorbed fatty acids bind calcium, allowing free oxalate to be absorbed from the colon
Explanation
With terminal ileal resection, unabsorbed bile acids and fatty acids reach the colon and bind luminal calcium. Calcium oxalate cannot form, so free oxalate is absorbed from the colon (enteric hyperoxaluria) and excreted, precipitating calcium oxalate stones. The same patient also loses the intrinsic factor dependent B12 uptake site, explaining the macrocytic anaemia. Citrate excess protects against, rather than causes, stones.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.