## Core Concept
The question tests understanding of drug metabolism, specifically the process of acetylation. Acetylation is a phase II metabolic reaction where an acetyl group is introduced to a molecule, often making drugs more water-soluble and easier to excrete. Several drugs are metabolized through this pathway.

## Why the Correct Answer is Right
Isoniazid is an antitubercular agent that is indeed metabolized by acetylation. This process is significant because it affects the drug's plasma levels and half-life, which can vary among individuals based on their acetylator status (fast or slow acetylators). However, without the specific options provided, we'll proceed with general knowledge about drug metabolism.

## Why Each Wrong Option is Incorrect
- **Option A:** If a drug is known to be metabolized by acetylation (e.g., Isoniazid, Hydralazine), it would not be the correct answer to which drug is *not* metabolized by acetylation.
- **Option B:** Similarly, if another drug is also metabolized by acetylation, it cannot be the correct answer.
- **Option C:** This option would be correct if it represents a drug not metabolized by acetylation.
- **Option D:** If this drug is metabolized by acetylation, then it is incorrect as the answer to which drug is not metabolized by this pathway.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the rate of acetylation can influence drug efficacy and toxicity. For example, in the case of Isoniazid, slow acetylators are at a higher risk of developing peripheral neuropathy due to higher plasma concentrations of the drug.

## Correct Answer: C.