A 40-year-old man underwent extensive resection of the terminal 80 cm of ileum for Crohn's disease. Two years later he passes bulky foul-smelling stools and recurrent calcium oxalate renal stones have formed. What is the mechanism of the stone formation?
- A Increased urinary excretion of unabsorbed vitamin B12 metabolites
- B Bacterial overgrowth deconjugates bile salts, increasing enteric oxalate synthesis
- C Loss of ileal bile salt absorption causes fat malabsorption, calcium binds fatty acids in the colon, and free oxalate is then absorbed from the colon ✓
- D Compensatory hyperparathyroidism raises filtered calcium load and promotes stone formation
Correct answer: C. Loss of ileal bile salt absorption causes fat malabsorption, calcium binds fatty acids in the colon, and free oxalate is then absorbed from the colon
Explanation
Ileal resection impairs bile salt reabsorption, causing bile salt depletion, fat malabsorption, and steatorrhoea. Intraluminal calcium is saponified by fatty acids, leaving dietary oxalate unbound; this free oxalate is hyperabsorbed across the colon and excreted in urine, forming calcium oxalate stones. This enteric hyperoxaluria pathway is the defining consequence tested here.
Reference: Sabiston Textbook of Surgery, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.