**Question:** All of the following are excreted in cystinuria except:

A. Cystine
B. Uric acid
C. Sulfate
D. Citrate

**Core Concept:** Cystinuria is a rare genetic disorder characterized by the abnormal excretion of cystine, a non-protein amino acid, leading to the formation of kidney stones and urinary tract infections.

**Why the Correct Answer is Right:** Among the options provided, the correct answer is **B. Uric acid**. Uric acid is a product of purine catabolism and is typically excreted in the urine. In cystinuria, the primary issue is with cystine excretion, not uric acid.

**Why Each Wrong Option is Incorrect:**

A. **Cystine**: Cystine is the correct answer because it is the amino acid primarily involved in cystinuria. It is a disulfide-linked dimer of cysteine and is not properly excreted in the urine, leading to stone formation and complications.

C. **Sulfate**: Sulfate is an essential anion that plays a crucial role in various physiological processes, including detoxification and bone mineralization. It is not the primary cause of cystinuria and is normally excreted in urine.

D. **Citrate**: Citrate is a urinary acidifier that helps prevent stone formation and is essential for maintaining kidney health. In cystinuria, the issue lies with cystine, not citrate, which is correctly excreted in the urine.

**Clinical Pearl:** Cystinuria is a prime example of a genetic disorder affecting the renal tubules, highlighting the importance of understanding the physiological processes involved in urine formation and stone prevention.

**Correct Answer:** D. Citrate

**Why Each Wrong Option is Incorrect:**

A. **Uric acid**: As mentioned earlier, uric acid is not the primary issue in cystinuria; it is a byproduct of purine catabolism and is normally excreted in urine.

B. **Phosphate**: Phosphate is a mineral ion involved in bone mineralization and is not directly linked to cystinuria. It is typically excreted in urine and does not contribute to stone formation in cystinuria.