What is the primary mechanism behind calcium oxalate nephrolithiasis in short bowel syndrome?

• A. Nephrolithiasis is multifactorial but primarily due to accelerated oxalate absorption in the colon
• B. Nephrolithiasis is due to dehydration alone
• C. Nephrolithiasis is caused by high calcium intake
• D. Nephrolithiasis results from reduced oxalate production by the gut

Answer: (A)

In short bowel syndrome, fat malabsorption drives calcium oxalate stone formation mainly through increased oxalate absorption in the colon. Unabsorbed fatty acids bind calcium in the gut, reducing calcium’s ability to chelate oxalate. This leaves more free oxalate available to be absorbed, and the colon becomes especially permeable to oxalate, leading to higher urinary oxalate (hyperoxaluria). The urine then becomes supersaturated with calcium oxalate, promoting stone formation. Dehydration from diarrhea can worsen stone risk by concentrating urine, but it is not the primary mechanism. Higher dietary calcium would actually help by binding oxalate in the gut, and reduced gut oxalate production is not the driving cause.