**Question:** In Cystinuria, all of the following amino acids are excreted in urine, except:-

A. Cystine
B. Arginine
C. Glutamine
D. Tyrosine

**Core Concept:**
Cystinuria is a rare genetic disorder characterized by the excessive excretion of cystine, arginine, and aspartate in urine due to a deficiency in the cystine transport protein. This results in the formation of renal calculi composed of these amino acids, leading to recurrent kidney stones and potential kidney damage.

**Why the Correct Answer is Right:**
In cystinuria, the primary defect lies in the transport of cystine, arginine, and aspartate across the renal tubule cells. Among the given options, D. Tyrosine is not involved in the formation of renal calculi in cystinuria. Tyrosine is a non-polar amino acid that does not cross the renal tubule membrane by the same mechanism as cystine and arginine, which are polar amino acids.

**Why Each Wrong Option is Incorrect:**
A. Cystine is the amino acid directly responsible for the formation of renal calculi in cystinuria due to its polar nature and inability to be reabsorbed back into the bloodstream.

B. Arginine is also a polar amino acid and is involved in the formation of renal calculi in cystinuria.

C. Glutamine is a polar amino acid but is not directly responsible for renal calculi formation in cystinuria, as it is not excreted in significant amounts along with cystine, arginine, and aspartate.

**Clinical Pearl:**
It is essential for medical students, particularly those studying urology or nephrology, to understand the specific amino acids involved in cystinuria pathogenesis. This knowledge helps in diagnosing the condition and tailoring appropriate management strategies for patients with this disorder.

**Correct Answer:**
D. Tyrosine