**Core Concept**
The rate-limiting step in norepinephrine synthesis is the conversion of tyrosine to L-DOPA by the enzyme tyrosine hydroxylase. This reaction is crucial for the production of catecholamines, including norepinephrine, dopamine, and epinephrine. Inhibiting this step can lead to a decrease in the synthesis of these neurotransmitters.

**Why the Correct Answer is Right**
Metyrosine, also known as alpha-methyltyrosine, is a competitive inhibitor of tyrosine hydroxylase. By binding to the enzyme, metyrosine prevents the conversion of tyrosine to L-DOPA, thereby inhibiting the rate-limiting step in norepinephrine synthesis. This mechanism is specific to metyrosine, making it a useful tool in the treatment of certain conditions, such as pheochromocytoma. The inhibition of tyrosine hydroxylase leads to a decrease in the production of catecholamines, which can help to control symptoms associated with excessive catecholamine production.

**Why Each Wrong Option is Incorrect**
**Option A:** Cocaine is a stimulant that increases the release of norepinephrine from nerve terminals by blocking its reuptake, rather than inhibiting its synthesis. It does not affect the rate-limiting step of norepinephrine synthesis.
**Option B:** Amphetamine, a central nervous system stimulant, increases the release of norepinephrine and other neurotransmitters by promoting their release from nerve terminals. It does not inhibit the synthesis of norepinephrine.
**Option D:** Bretylium is an antihypertensive agent that works by blocking the release of norepinephrine from nerve terminals, rather than inhibiting its synthesis.

**Clinical Pearl / High-Yield Fact**
Metyrosine can be used to treat pheochromocytoma, a rare tumor of the adrenal gland that produces excessive amounts of catecholamines. By inhibiting the synthesis of norepinephrine, metyrosine can help to control symptoms associated with excessive catecholamine production, such as hypertension and tachycardia.

**✓ Correct Answer: C. Metyrosine**