Which of the following is not a broad spectrum antibiotic?

Solution

Chemistry ma’am · 3 steps

Penicillin G is the classic narrow spectrum antibiotic; the other three are on NCERT's broad spectrum list.

1. Two kinds. A broad spectrum antibiotic kills or inhibits a wide range of bacteria, both Gram-positive and Gram-negative. A narrow spectrum antibiotic works mainly against one group, either Gram-positive or Gram-negative.

   Antibiotic	Acts mainly on	Spectrum
   Vancomycin	listed by NCERT as wide range	broad
   Ampicillin	Gram-positive and many Gram-negative	broad
   Ofloxacin	Gram-positive and Gram-negative	broad
   Penicillin G	mainly Gram-positive	narrow

2. Why ampicillin differs. Ampicillin is a modified penicillin. The extra amino group lets it get into many Gram-negative bacteria, which Penicillin G cannot do well. That change turns a narrow spectrum drug into a broad spectrum one.
3. So. Penicillin G is the only narrow spectrum antibiotic here, so the answer is (d).

The idea to keep. For the exam, remember NCERT's two lists: broad spectrum are ampicillin, amoxycillin, chloramphenicol, vancomycin and ofloxacin. Penicillin G is the standard example of narrow spectrum.

Answer from the NTA final answer key. Solution by PrepWiser.