Cystinuria is primarily characterized by what condition?

• A. Excretion of elevated leucine
• B. Defective renal reabsorption of cystine
• C. Excess methionine in urine
• D. Increased tyrosine levels

Answer: (B)

Cystinuria is primarily characterized by the defective renal reabsorption of cystine. This condition is a genetic disorder caused by mutations in the transporters responsible for the reabsorption of certain amino acids, including cystine, in the kidneys. As a result, individuals with cystinuria have a reduced ability to reabsorb cystine, leading to its increased excretion in the urine.

Cystine is an amino acid that forms through the dimerization of two cysteine molecules, and its elevated levels in urine can lead to the formation of kidney stones, as cystine is relatively insoluble. This condition can result in significant health issues, including renal colic and urinary tract obstruction due to stone formation, underscoring the importance of recognizing and managing cystinuria effectively.

The other conditions associated with the other options do not relate directly to cystinuria. Elevated leucine excretion is relevant in different metabolic disorders, methionine levels pertain to different amino acid metabolism issues, and increased tyrosine levels may be associated with other metabolic disorders. Understanding cystinuria's mechanism enables better diagnosis and management of the condition.