**Core Concept**
Cystinuria is a rare inherited disorder characterized by the abnormal reabsorption of cystine and other dibasic amino acids (lysine, arginine, and ornithine) in the kidneys, leading to their excessive excretion in the urine. This results in the formation of cystine stones, which can cause severe kidney damage if left untreated.

**Why the Correct Answer is Right**
Cystinuria is caused by mutations in the SLC3A1 and SLC7A9 genes, which encode for the rBAT and b0,+AT subunits of the heterodimeric cystine transporter in the renal proximal tubule. The impaired reabsorption of cystine and other dibasic amino acids leads to their accumulation in the urine, promoting the formation of cystine stones. The pathophysiology of cystinuria is closely related to the impaired function of this cystine transporter.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because cystine stones are a hallmark of cystinuria, and their presence is a key diagnostic criterion.
* **Option B:** This option is incorrect because the SLC3A1 and SLC7A9 genes are responsible for encoding the cystine transporter, and mutations in these genes are the primary cause of cystinuria.
* **Option C:** This option is incorrect because the renal proximal tubule is the primary site of cystine reabsorption, and impaired function of the cystine transporter in this region leads to cystinuria.

**Clinical Pearl / High-Yield Fact**
Cystinuria is an autosomal recessive disorder, meaning that affected individuals must inherit two mutated alleles (one from each parent) to express the condition.

**Correct Answer:** A.