Question

All of the following are excreted in excess in cystinuria except?

• A. Cystine
• B. Cysteine
• C. Arginine
• D. Ornithine

Hint:

Remember COLA, and note the dimer cystine versus the single amino acid cysteine.

Answer 1

The Correct Option is B

Step 1: Cystinuria is an autosomal recessive defect of the renal tubular and intestinal transporter that normally reabsorbs the dibasic amino acids. The affected amino acids are cystine, ornithine, lysine and arginine (remembered as COLA).

Step 2: Because their reabsorption fails, cystine, ornithine, lysine and arginine all appear in increased amounts in the urine. Cystine is poorly soluble and precipitates as hexagonal crystals, forming kidney stones.

Step 3: Cysteine, the reduced single-amino-acid form, is not the amino acid handled by this transporter; the transporter and the disease involve cystine (two cysteines joined by a disulfide bond), not free cysteine. So the exception is cysteine.

Step 4: Therefore arginine and ornithine are correctly excreted in excess, while cysteine is the one that is not the characteristic urinary amino acid here.