**Core Concept**
Cystinuria is a rare genetic disorder characterized by the excessive excretion of cystine and other dibasic amino acids (lysine, arginine, and ornithine) in the urine. This occurs due to a defect in the reabsorption of these amino acids in the kidneys.

**Why the Correct Answer is Right**
Cystinuria results from mutations in the SLC3A1 and SLC7A9 genes, which encode proteins responsible for the transport of cystine and dibasic amino acids in the kidneys. The defective reabsorption leads to an accumulation of these amino acids in the urine, causing cystine stones and other complications. Cystine, lysine, arginine, and ornithine are all dibasic amino acids that are affected in cystinuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystine is a dibasic amino acid that is excreted in excess in cystinuria, so this option is incorrect.
**Option B:** Lysine is a dibasic amino acid that is also excreted in excess in cystinuria, making this option incorrect as well.
**Option C:** Arginine is another dibasic amino acid that is affected in cystinuria, so this option is also incorrect.

**Clinical Pearl / High-Yield Fact**
Cystinuria is an autosomal recessive disorder, meaning that a person must inherit two defective copies of the gene (one from each parent) to develop the condition. Early diagnosis and treatment can help prevent cystine stones and other complications.

**Correct Answer: D.**