**Question:** Hyperammonemia type I is due to deficiency of

A. Ornithine Transcarbamylase
B. Alanine Transaminase
C. Argininosuccinate Synthase
D. Citrulline Synthase

**Core Concept:**
Hyperammonemia is a condition characterized by elevated blood ammonia levels. In humans, ammonia is primarily removed from the blood through the urea cycle, a series of enzyme-catalyzed reactions in the liver. The urea cycle has eight enzymes, collectively known as the urea cycle enzymes, which are essential for ammonia detoxification. These enzymes are involved in the conversion of ammonia into less toxic compounds, ultimately leading to urea production.

**Why the Correct Answer is Right:**
Type I hyperammonemia is caused by deficiency of a specific urea cycle enzyme, specifically, Argininosuccinate Synthase. Argininosuccinate Synthase is an enzyme responsible for the first committed step in the urea cycle, catalyzing the formation of argininosuccinate from citrulline and fumarate. When this enzyme is deficient, the urea cycle is impaired, leading to hyperammonemia.

**Why Each Wrong Option is Incorrect:**

A. Ornithine Transcarbamylase deficiency: This enzyme is involved in the second step of the urea cycle, catalyzing the formation of citrulline from carbamoyl phosphate and ornithine. Its deficiency would lead to impaired generation of ornithine, not hyperammonemia.

B. Alanine Transaminase: This enzyme is involved in the transamination process, specifically between alanine and aspartate, with the production of pyruvate and glutamate. Its deficiency would not cause hyperammonemia directly.

C. Citrullinase: Citrullinase is an enzyme that cleaves citrulline back into arginine and ornithine, not directly involved in the urea cycle. Its deficiency would not cause hyperammonemia.

D. Citrulline Synthase: Citrulline Synthase is an enzyme involved in the conversion of argininosuccinate to citrulline, which is the next step in the urea cycle. Its deficiency would lead to impaired formation of citrulline, not hyperammonemia.

**Clinical Pearl:**
Understanding the urea cycle and its enzymes is crucial for recognizing and diagnosing hyperammonemia, as it helps in understanding the underlying etiology and appropriate treatment options. Adequate ammonia detoxification is crucial for maintaining normal brain function, as ammonia is neurotoxic in high concentrations.

**Correct Answer Explanation:**

Type I hyperammonemia is caused by the deficiency of Argininosuccinate Synthase (Citrullinase), which catalyzes the formation of argininosuccinate from citrulline and fumarate, the third step in the urea cycle. The deficiency of Argininosuccinate Synthase leads to impaired synthesis of argininosuccinate, resulting in reduced synthesis of arginine, ornithine