**Core Concept**
Nitric oxide (NO) is a crucial signaling molecule involved in various physiological and pathological processes, including vasodilation, neurotransmission, and immune response. Its production is tightly regulated by the enzyme nitric oxide synthase (NOS), which catalyzes the conversion of a specific amino acid to citrulline and NO.

**Why the Correct Answer is Right**
Nitric oxide is synthesized from the amino acid L-arginine through the action of nitric oxide synthase (NOS). The enzyme catalyzes the oxidation of L-arginine to citrulline and NO, with the help of NADPH and oxygen as cofactors. This process is crucial for the production of NO in various cell types, including endothelial cells, neurons, and macrophages.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is a common distractor, but it is not the correct amino acid involved in NO synthesis.

**Option B:** This option is a plausible choice, but it is not the primary amino acid used by NOS for NO production.

**Option C:** This option is incorrect, as it is not directly related to the synthesis of nitric oxide.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the production of NO is tightly regulated by various factors, including calcium levels, protein kinase G activity, and the availability of L-arginine. An imbalance in NO production can lead to various diseases, such as atherosclerosis, hypertension, and sepsis.

**Correct Answer: C. L-arginine**