In steatorrhea, what mechanistic change increases the risk of calcium oxalate nephrolithiasis?

• A. Steatorrhea reduces oxalate absorption
• B. Oxalate binds calcium in the gut and is excreted in stool.
• C. Calcium binds fatty acids, leaving oxalate free to be absorbed and cause oxalate nephropathy
• D. Oxalate is degraded by pancreatic enzymes

Answer: (C)

When fat is malabsorbed, there are plenty of fatty acids in the gut lumen. Calcium normally binds oxalate, forming insoluble calcium oxalate that is excreted in stool. But the excess fatty acids drive calcium to bind them instead, forming calcium soaps and leaving less free calcium available to bind oxalate. With less calcium to trap oxalate in the gut, more oxalate is absorbed into the bloodstream. The result is higher urinary oxalate (hyperoxaluria), which combines with calcium in the urine to form calcium oxalate stones. So the key change is that calcium binds fatty acids, leaving oxalate free to be absorbed and promote stone formation.