Most common enzyme deficient in urea cycle:
**Core Concept**
The urea cycle is a crucial metabolic pathway that converts ammonia into urea, primarily taking place in the liver. The cycle involves a series of enzyme-catalyzed reactions that require precise regulation to maintain nitrogen homeostasis. A deficiency in any of these enzymes can lead to hyperammonemia and potentially life-threatening complications.
**Why the Correct Answer is Right**
The most common enzyme deficient in the urea cycle is Carbamoyl Phosphate Synthetase I (CPS I). This enzyme is responsible for the first committed step of the cycle, catalyzing the condensation of ammonia and bicarbonate to form carbamoyl phosphate. A deficiency in CPS I leads to a condition known as N-Acetylglutamate Deficiency, which results in impaired urea synthesis and increased ammonia levels in the blood. This condition is often caused by mutations in the CPS1 gene and can be inherited in an autosomal recessive pattern.
**Why Each Wrong Option is Incorrect**
* **Option A:** Ornithine Transcarbamylase (OTC) deficiency is a relatively common urea cycle disorder, but it is not the most common enzyme deficiency. OTC deficiency leads to impaired conversion of carbamoyl phosphate to citrulline.
* **Option B:** Argininosuccinate Synthetase (ASS) deficiency is another urea cycle disorder, but it is less common than CPS I deficiency. ASS deficiency leads to impaired conversion of citrulline to argininosuccinate.
* **Option C:** Arginase deficiency is a rare urea cycle disorder that leads to impaired conversion of arginine to urea. While it is a significant deficiency, it is not the most common enzyme deficiency in the urea cycle.
**Clinical Pearl / High-Yield Fact**
Urea cycle disorders can be challenging to diagnose, and a high index of suspicion is necessary. A key clinical clue is the presence of hyperammonemia, often accompanied by neurological symptoms such as seizures, vomiting, and altered mental status. Early recognition and treatment are crucial to prevent long-term complications and improve outcomes.
**Correct Answer: D. Carbamoyl Phosphate Synthetase I (CPS I).**