Q.85 Match the antibiotics given in Group I with appropriate targets from Group II.
Group I
(P) Nalidixic acid
(Q) Tetracycline
(R) Erythromycin
(S) Rifampin
Group II
(I) RNA polymerase
(II) DNA gyrase
(III) DNA polymerase
(IV) 50 S ribosomal subunit
(V) Aminoacyl tRNA
(A) P-III, Q-IV, R-V, S-I
(B) P-V, Q-I, R-IV, S-II
(C) P-II, Q-V, R-IV, S-I
(D) P-II, Q-V, R-I, S-IV
Antibiotics target specific bacterial components to halt growth or kill pathogens. In this antibiotic targets matching MCQ, Group I lists nalidixic acid (P), tetracycline (Q), erythromycin (R), and rifampin (S). Group II includes RNA polymerase (I), DNA gyrase (II), DNA polymerase (III), 50S ribosomal subunit (IV), and aminoacyl tRNA (V). Matching them correctly tests your grasp of molecular microbiology—essential for exams like NEET PG, USMLE, or biotech courses.
Correct Answer: Option (C) P-II, Q-V, R-IV, S-I
Option (C) pairs each antibiotic with its precise bacterial target:
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P (Nalidixic acid) – II (DNA gyrase): Nalidixic acid, a quinolone precursor, inhibits DNA gyrase (topoisomerase II). This enzyme relieves supercoiling during DNA replication, so blocking it halts bacterial DNA synthesis. It doesn’t affect eukaryotic gyrase due to structural differences.
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Q (Tetracycline) – V (Aminoacyl tRNA): Tetracyclines bind the 30S ribosomal subunit, preventing aminoacyl-tRNA from entering the A-site. This blocks protein synthesis at elongation—broad-spectrum action against Gram-positive and negative bacteria.
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R (Erythromycin) – IV (50S ribosomal subunit): A macrolide, erythromycin binds the 50S subunit’s exit tunnel, inhibiting translocation during translation. It targets Gram-positive bacteria effectively, sparing mammalian ribosomes.
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S (Rifampin) – I (RNA polymerase): Rifampin binds bacterial RNA polymerase’s β-subunit, blocking mRNA chain elongation after ~2-3 nucleotides. Crucial for TB treatment, it spares human RNA polymerase.
This matching reflects standard pharmacology: quinolones on gyrase, tetracyclines on tRNA binding, macrolides on 50S, and rifamycins on transcription.
Why Other Options Are Incorrect
Understanding wrong pairings highlights common pitfalls in antibiotic targets matching MCQ prep.
Option (A): P-III, Q-IV, R-V, S-I
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Wrong: Nalidixic acid (P) doesn’t target DNA polymerase (III)—that’s acyclovir for viruses or fidaxomicin indirectly. Tetracycline (Q) skips 50S (IV, macrolide/chloramphenicol target) for tRNA. Erythromycin (R) misses 50S for tRNA (V). Rifampin (S-I) is correct, but others fail.
Option (B): P-V, Q-I, R-IV, S-II
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Wrong: Nalidixic acid (P) ignores tRNA (V, tetracycline’s site) for gyrase. Tetracycline (Q) doesn’t hit RNA polymerase (I). Erythromycin (R-IV) is right. Rifampin (S) skips RNA polymerase for gyrase (II, quinolone target).
Option (D): P-II, Q-V, R-I, S-IV
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Wrong: Nalidixic acid (P-II) and tetracycline (Q-V) match perfectly. But erythromycin (R) targets 50S (IV), not RNA polymerase (I). Rifampin (S) hits RNA polymerase (I), not 50S (IV).
| Option | P (Nalidixic acid) | Q (Tetracycline) | R (Erythromycin) | S (Rifampin) | Correct? |
|---|---|---|---|---|---|
| (A) | III (Wrong) | IV (Wrong) | V (Wrong) | I (Correct) | No |
| (B) | V (Wrong) | I (Wrong) | IV (Correct) | II (Wrong) | No |
| (C) | II (Correct) | V (Correct) | IV (Correct) | I (Correct) | Yes |
| (D) | II (Correct) | V (Correct) | I (Wrong) | IV (Wrong) | No |
Key Takeaways for Antibiotic Targets Matching MCQ Success
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Memorize by class: Quinolones (gyrase), tetracyclines (tRNA/30S), macrolides (50S), rifamycins (RNA pol).
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Clinical relevance: Nalidixic for UTIs, tetracycline for acne/chlamydia, erythromycin for respiratory infections, rifampin for TB/meningitis.
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Exam tip: Distinguish prokaryotic targets—eukaryotes lack gyrase, have different ribosomes/RNA pol.
Practice more antibiotic targets matching MCQ to ace your microbiology or pharmacology exams!
