**Core Concept**
Neutrophil extracellular traps (NETs) are networks of extracellular fibers, primarily composed of DNA, that are released by neutrophils in response to certain infections. The formation of NETs involves a process called NETosis, which is mediated by the enzyme peptidyl arginine deiminase 4 (PAD4). PAD4 catalyzes the conversion of arginine residues to citrulline, leading to the degradation of histones and the subsequent release of DNA.

**Why the Correct Answer is Right**
Citrulline is the key amino acid involved in NET formation. PAD4, the enzyme responsible for NETosis, specifically targets arginine residues and converts them to citrulline. This post-translational modification of histones leads to the unwinding of chromatin, allowing for the release of DNA into the extracellular space. The citrullination of histones is a critical step in the formation of NETs, and citrulline is the amino acid that results from this process.

**Why Each Wrong Option is Incorrect**
**Option A:** Leucine is a branched-chain amino acid that plays a role in protein synthesis and metabolism but is not directly involved in the formation of NETs.
**Option B:** Methionine is another essential amino acid that is important for various bodily functions, including the synthesis of glutathione, but it is not involved in NETosis.
**Option D:** Valine is a branched-chain amino acid that, like leucine, is involved in protein synthesis and metabolism but has no direct role in NET formation.

**Clinical Pearl / High-Yield Fact**
NETs are a crucial defense mechanism against certain infections, particularly those caused by bacteria such as Staphylococcus aureus and Candida albicans. However, excessive NET formation can contribute to tissue damage and disease, highlighting the complex interplay between NETs and the host immune response.

**✓ Correct Answer: C. Citrulline**