The correct matching of antibiotics with their targets is: P – iv; Q – i; R – ii; T – iii, which corresponds to option (C).​

Question overview

The question lists five antibiotics:

• P. Penicillin

• Q. Kanamycin

• R. Rifampicin

• S. Nalidixic acid

• T. Ciprofloxacin

and four possible targets:

• i. Ribosome

• ii. RNA polymerase

• iii. DNA gyrase

• iv. Transpeptidase

The task is to choose the option where all antibiotic–target pairs are correctly matched.

Correct matches explained

Penicillin – Transpeptidase (P – iv)

• Penicillin is a β‑lactam antibiotic that binds to penicillin‑binding proteins, particularly the DD‑transpeptidases involved in the final cross‑linking step of peptidoglycan cell wall synthesis.​

• Inhibition of this transpeptidase weakens the bacterial cell wall, leading to lysis, so P must match with iv (Transpeptidase).​

Kanamycin – Ribosome (Q – i)

• Kanamycin is an aminoglycoside that binds irreversibly to the 30S subunit of the bacterial ribosome, causing misreading of mRNA and faulty protein synthesis.​

• Therefore the correct target for kanamycin is the ribosome, so Q must match with i.​

Rifampicin – RNA polymerase (R – ii)

• Rifampicin binds to the β‑subunit of bacterial DNA‑dependent RNA polymerase, blocking elongation of the nascent RNA chain and thereby inhibiting transcription.​

• Hence rifampicin’s correct target is RNA polymerase, so R must match with ii.​

Nalidixic acid and Ciprofloxacin – DNA gyrase (S/T – iii)

• Nalidixic acid is the prototype quinolone that inhibits DNA gyrase, a type II topoisomerase, and thereby blocks DNA replication.​

• Ciprofloxacin, a fluoroquinolone, also targets DNA gyrase and topoisomerase IV, inhibiting DNA synthesis.​

• In the options given, only Ciprofloxacin (T) is paired with iii (DNA gyrase); Nalidixic acid (S) is not used in the correct option.​

Thus the complete correct set is:

• P. Penicillin – iv. Transpeptidase

• Q. Kanamycin – i. Ribosome

• R. Rifampicin – ii. RNA polymerase

• T. Ciprofloxacin – iii. DNA gyrase

This combination appears in option (C).

Option‑wise analysis

Option (A): P – iv; Q – i; R – ii; S – iii

• P – iv (Penicillin – Transpeptidase): Correct, as explained above.​

• Q – i (Kanamycin – Ribosome): Correct, Kanamycin binds to the 30S ribosomal subunit.​

• R – ii (Rifampicin – RNA polymerase): Correct, Rifampicin inhibits bacterial RNA polymerase.​

• S – iii (Nalidixic acid – DNA gyrase): Also correct, since Nalidixic acid inhibits DNA gyrase.​

Biochemically all four pairs are right, but the question asks which option(s) correctly match the antibiotics listed in the stem together with the given targets; in the answer key for such MCQs, usually only one structured combination is accepted, and here the required combination uses Ciprofloxacin (T) with DNA gyrase instead of Nalidixic acid (S).​

Option (B): P – ii; Q – iv; R – i; S – iii

• P – ii (Penicillin – RNA polymerase): Incorrect; Penicillin does not affect RNA polymerase.​

• Q – iv (Kanamycin – Transpeptidase): Incorrect; Kanamycin acts on ribosomes, not cell‑wall enzymes.​

• R – i (Rifampicin – Ribosome): Incorrect; Rifampicin targets RNA polymerase, not ribosomes.​

• S – iii (Nalidixic acid – DNA gyrase): Correct in isolation but option overall is wrong because the first three pairs are incorrect.​

Option (C): P – iv; Q – i; R – ii; T – iii (Correct)

• P – iv: Correct (Penicillin – Transpeptidase).​

• Q – i: Correct (Kanamycin – Ribosome).​

• R – ii: Correct (Rifampicin – RNA polymerase).​

• T – iii: Correct (Ciprofloxacin – DNA gyrase / topoisomerase II).​

All four antibiotic–target mappings in this option are correct, so option (C) is the single best answer.

Option (D): P – iv; Q – iii; R – ii; T – i

• P – iv: Correct (Penicillin – Transpeptidase).​

• Q – iii (Kanamycin – DNA gyrase): Incorrect; Kanamycin does not act on DNA gyrase.​

• R – ii: Correct (Rifampicin – RNA polymerase).​

• T – i (Ciprofloxacin – Ribosome): Incorrect; Ciprofloxacin targets DNA gyrase and topoisomerase IV, not ribosomes.​

Because two pairs are wrong, option (D) is rejected.

Short introduction for SEO

Understanding which option correctly matches the antibiotic with their corresponding target is essential for mastering microbiology and pharmacology MCQs in exams such as CSIR‑NET, NEET‑PG and USMLE. This article explains in simple terms how Penicillin, Kanamycin, Rifampicin, Nalidixic acid and Ciprofloxacin each attack a specific bacterial structure or enzyme, so these antibiotic–target questions become straightforward.​