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

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

**Core Concept:** Cystinuria is a genetic disorder characterized by the abnormal excretion of cystine, an amino acid, in the urine. This results in the formation of kidney stones and sometimes intestinal obstruction due to the accumulation of cystine crystals within the urinary tract.

**Why the Correct Answer is Right:** Uric acid (A), urate (B), and sulfate (D) are all anions, which are negatively charged ions that can be transported across cell membranes by specific transport proteins. However, cystine (C) is a disulfide-linked dimer of cysteine, a different amino acid. In cystinuria, the impaired reabsorption of cystine in the renal tubules leads to its excretion in the urine.

**Why Each Wrong Option is Incorrect:**

Option A (Uric acid) and Option B (Urate) are both transported to the urine due to their negative charge, unlike cystine, which is not transported and excreted due to its unique structure as a disulfide-linked dimer of cysteine.

Option D (Sulfate) is also transported to the urine due to its negative charge, but cystine is not, which makes cystine the correct answer.

**Clinical Pearl:** Cystinuria is a rare condition that can lead to severe morbidity and potential mortality due to kidney stone formation and intestinal obstruction. Early diagnosis and management are crucial to prevent complications and improve the patient's quality of life.

**Correct Answer:** D. Sulfate

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**Core Concept:** Cystine is a disulfide-linked dimer of cysteine, an amino acid. In cystinuria, cystine is not reabsorbed in the renal tubules, leading to its excretion in the urine.

**Why the Correct Answer is Right:** The correct answer, sulfate (D), is a negatively charged anion that is reabsorbed in the renal tubules due to its ability to interact with the negatively charged renal tubular cells. In contrast, cystine (C) is not transported due to its unique structure as a disulfide-linked dimer of cysteine and is excreted in the urine.

**Why Each Wrong Option is Incorrect:**

Option A (Uric acid) is a product of purine catabolism and is reabsorbed due to its negative charge in the renal tubules.

Option B (Urate) is the end product of purine catabolism and is reabsorbed in the renal tubules due to its negative charge.

Option C (Cystine) is a disulfide-linked dimer of cysteine and is excreted in the urine due to its unique structure that prevents reabsorption by the renal tubular cells.

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**Clinical Pearl:** Cystinuria is a rare genetic disorder that results in the abnormal excretion