Which of the following drugs is a folic acid antagonist:

Correct Answer: C

Rationale: Co-trimoxazole inhibits folic acid synthesis via sequential blockade.

Correct Answer: C

Rationale: Tetracyclines cause permanent dental staining in children due to deposition in developing teeth.

Correct Answer: A

Rationale: Probenecid inhibits renal excretion of penicillin, prolonging its action.

Correct Answer: D

Rationale: Nystatin is used topically or orally for local effects, not systemically.

Correct Answer: B

Rationale: Niclosamide is used for tapeworm infestations.