**Question:** In argininosuccinase deficiency, what should be supplemented to continue the urea cycle?

**Core Concept:**
Argininosuccinase deficiency is a rare genetic disorder caused by the deficiency of argininosuccinate lyase enzyme, which is essential for maintaining the proper functioning of the urea cycle. The urea cycle is crucial for nitrogen excretion from the body, as it converts ammonia, a toxic substance, into urea, which can be excreted via urine.

**Why the Correct Answer is Right:**
In argininosuccinase deficiency, the deficiency of argininosuccinate lyase enzyme disrupts the urea cycle, leading to the accumulation of argininosuccinate and the accumulation of ammonia in the body. The correct answer is **Option C, Ornithine**. Ornithine is a precursor to arginine, which is required for the synthesis of argininosuccinate. By supplementing ornithine, the deficiency can be bypassed, allowing the urea cycle to proceed despite the lack of functional argininosuccinate lyase enzyme.

**Why Each Wrong Option is Incorrect:**

**Option A (Arginine):**
Arginine is another precursor of the urea cycle, but it is not involved in the synthesis of argininosuccinate. Supplementing arginine will not help in bypassing the deficiency of argininosuccinate lyase enzyme.

**Option B (Argininosuccinate):**
Argininosuccinate is the substrate for argininosuccinate lyase, which is deficient in argininosuccinate lyase deficiency. Supplementing argininosuccinate would exacerbate the problem by further disrupting the urea cycle.

**Option D (Urea):**
Urea is the end product of the urea cycle and not an essential precursor or intermediate in the pathway affected by argininosuccinate lyase deficiency. Supplementing urea would not help in bypassing the deficiency of argininosuccinate lyase enzyme.

**Clinical Pearl:**
In argininosuccinate lyase deficiency, supplementing ornithine is the key to bypassing the deficiency and allowing the urea cycle to proceed despite the lack of functional argininosuccinate lyase enzyme.

**Correct Answer Explanation:**
In argininosuccinate lyase deficiency, the primary issue is the deficiency of argininosuccinate lyase enzyme, which is involved in the synthesis of argininosuccinate. By supplementing ornithine, the urea cycle can be bypassed, allowing the body to produce arginine from ornithine, which can then be converted into argininosuccinate and eventually urea. This helps in excreting ammonia, reducing its toxic effects and preventing neurological damage.