**Core Concept**
Acetylation is a metabolic pathway primarily mediated by the enzyme N-acetyltransferase (NAT), which converts certain drugs into their acetylated metabolites. This process is highly variable among individuals and is a key determinant of drug response and toxicity, especially in drugs like isoniazid (INH), dapsone, and procainamide.

**Why the Correct Answer is Right**
Metoclopramide is metabolized primarily by the cytochrome P450 system (especially CYP2D6 and CYP3A4), not by acetylation. In contrast, dapsone, procainamide, and isoniazid (INH) are all substrates of the N-acetyltransferase enzyme. Isoniazid is acetylated to acetylisoniazid, dapsone to N-acetyl-dapsone, and procainamide to N-acetylprocainamide. Thus, metoclopramide does not undergo acetylation, making it the correct exception.

**Why Each Wrong Option is Incorrect**
Option A: Dapsone is acetylated by NAT2, forming N-acetyl-dapsone, which is a major metabolic pathway.
Option C: Procainamide undergoes N-acetylation to form N-acetylprocainamide, a metabolite with different pharmacological activity.
Option D: Isoniazid (INH) is extensively acetylated by NAT2, forming acetylisoniazid, which is a key metabolic route.

**Clinical Pearl / High-Yield Fact**
Patients with poor acetylator phenotype (slow acetylators) are at higher risk of INH toxicity due to accumulation of parent drug, while fast acetylators may have reduced efficacy. This polymorphism is clinically significant in managing INH therapy and dapsone-related side effects.

✓ Correct Answer: B. Metoclopramide