Sulfanilamide is the primary growth factor analog among the options, mimicking para-aminobenzoic acid (PABA) essential for bacterial folate synthesis. This competitive inhibition disrupts bacterial growth by blocking a key metabolic pathway. The correct answer is (C) Sulfanilamide.​

Correct Answer

The correct choice is (C) Sulfanilamide. It functions as a structural analog of PABA, a growth factor required by bacteria for folic acid production, leading to bacteriostatic effects.​

Option Breakdown

• (A) 5-Fluorouracil: This is a pyrimidine analog used in anticancer therapy, inhibiting thymidylate synthase and DNA/RNA synthesis in eukaryotic cells, not primarily as an antimicrobial growth factor mimic.​

• (B) Isoniazid: Targets mycolic acid synthesis in Mycobacterium tuberculosis by inhibiting InhA enzyme after activation by KatG, acting as a prodrug rather than a growth factor analog.​

• (C) Sulfanilamide: Acts as a PABA analog, competitively inhibiting dihydropteroate synthase (folP) in the folate pathway, starving bacteria of tetrahydrofolate needed for nucleotide synthesis—a classic growth factor antagonist example.​

• (D) Tetracycline: Binds to the 30S ribosomal subunit, inhibiting protein synthesis by blocking tRNA binding, without mimicking any growth factor.​

Mechanism Insights

Growth factor analogs like sulfanilamide exploit bacterial auxotrophy for essential metabolites such as folate precursors, unavailable via uptake in many microbes. Sulfonamides bind pteroyltransferase, halting dihydropteroic acid formation critical for thymine and purine production. This selective toxicity spares humans, who obtain folate preformed in diet.​