In argininosuccinase deficiency, what should be supplemented to continue the urea cycle ?
**Core Concept**
Argininosuccinase deficiency is a rare genetic disorder that affects the urea cycle, leading to an accumulation of toxic intermediates. The urea cycle is a critical process by which the body converts ammonia into urea for excretion. In argininosuccinase deficiency, the enzyme argininosuccinase is deficient, causing a block in the cycle at the step where argininosuccinate is converted to arginine and fumarate.
**Why the Correct Answer is Right**
To bypass the block in the urea cycle, arginine supplementation is necessary. Arginine serves as a precursor to ornithine, which is then converted to citrulline, allowing the cycle to continue. This supplementation is essential to prevent the accumulation of toxic intermediates and to maintain nitrogen balance in the body. The correct answer is arginine because it is the key molecule that helps to bypass the defective enzyme and maintain the integrity of the urea cycle.
**Why Each Wrong Option is Incorrect**
* **Option A:** Citrulline supplementation would not be sufficient to bypass the block in the urea cycle because citrulline is downstream of the defective enzyme. While citrulline is an important intermediate in the cycle, it does not address the underlying deficiency.
* **Option B:** Fumarate supplementation would not be effective in bypassing the block in the urea cycle. Fumarate is an intermediate that is produced in the cycle, but it does not provide a means to bypass the defective enzyme.
* **Option D:** Ornithine supplementation would not be sufficient to bypass the block in the urea cycle because ornithine is a downstream product of the defective enzyme. Ornithine is essential for the cycle, but it does not address the underlying deficiency.
**Clinical Pearl / High-Yield Fact**
In urea cycle disorders, including argininosuccinase deficiency, supplementation with citrulline and ornithine can help to bypass the block and maintain the integrity of the cycle. However, arginine supplementation remains the most effective treatment for argininosuccinase deficiency.
**Correct Answer:** C. Arginine.