**Question:** All are alkylating agents, except:

A. Ethambutol
B. Isoniazid
C. Rifampicin
D. Streptomycin

**Core Concept:** Alkylating agents are a class of drugs that modify nucleic acids, particularly DNA, by adding alkyl groups (a type of chemical compound) to the nitrogenous bases. This can lead to impaired DNA replication and transcription, ultimately causing cell cycle arrest, apoptosis, or mutagenesis, depending on the extent of damage. Alkylating agents are commonly used as chemotherapeutic agents in cancer treatment.

**Why the Correct Answer is Right:** Ethambutol, Isoniazid, Rifampicin, and Streptomycin are all anti-tubercular drugs used in the treatment of tuberculosis (TB). While they all target Mycobacterium tuberculosis, they work through different mechanisms:

- Ethambutol and Isoniazid inhibit mycolic acid synthesis and fatty acid biosynthesis, respectively, leading to cell wall disruption and death of the bacteria.
- Rifampicin is an inhibitor of RNA polymerase, blocking the synthesis of bacterial RNA and thus impairing cell growth and division.
- Streptomycin targets the 16S rRNA, leading to inhibition of protein synthesis and bacteriostasis.

All three explanations emphasize that these drugs are not alkylating agents, which is why option D (Streptomycin) is correct.

**Why Each Wrong Option is Incorrect:**

A. **Ethambutol**: Although Ethambutol is effective against TB, it does not act as an alkylating agent. It inhibits mycolic acid synthesis, leading to cell wall disruption, but does not involve adding alkyl groups to nucleic acids.

B. **Isoniazid**: Isoniazid is an inhibitor of the enzyme InhA, which is a part of the InhA-NAD complex involved in fatty acid synthesis. It does not act as an alkylating agent, unlike the other options.

C. **Rifampicin**: Rifampicin is an inhibitor of RNA polymerase, blocking RNA synthesis in bacteria. This mechanism does not involve alkylating agents, which are nucleophilic agents that add alkyl groups to nucleic acids.

**Clinical Pearls:** Alkylating agents are essential components of chemotherapy regimens for various cancers and certain blood disorders. Understanding the mechanisms of action of these drugs is crucial for any medical professional, especially considering the increasing prevalence of drug-resistant TB strains and the need for alternative treatments.