What is the underlying genetic defect in cystinuria?

Cystinuria is primarily caused by a defect in the renal tubular transport of certain amino acids, particularly cystine and the dibasic amino acids lysine, arginine, and ornithine. This defect leads to the impaired reabsorption of these amino acids in the proximal tubules of the kidneys, resulting in their accumulation in the urine. Consequently, cystine can precipitate in the renal tubules and form stones, leading to renal colic and other complications.

The genetic basis of cystinuria involves mutations in the SLC3A1 or SLC7A9 genes, which encode for the transporters responsible for the reabsorption of cysteine and the dibasic amino acids. Therefore, the underlying genetic defect in cystinuria is accurately described as a defective renal transport of amino acids, leading to the condition's characteristic features.