Antibiotic spectrum basics MCQs: 15 Questions with Answers

Correct answer: C. Vancomycin

Vancomycin is effective mainly against Gram‑positive organisms, making it a narrow‑spectrum antibiotic.

Correct answer: C. Tetracyclines

Tetracyclines inhibit protein synthesis by binding to the 30S ribosomal subunit and exhibit broad-spectrum activity against a wide variety of gram-positive, gram-negative, and atypical pathogens.

Correct answer: B. It is effective against a specific, limited group of microorganisms

Narrow-spectrum antibiotics target a specific group of bacteria (such as primarily gram-positive organisms), thereby minimizing disruption to the normal host microbiota compared to broad-spectrum agents.

Correct answer: C. Benzylpenicillin (Penicillin G)

Benzylpenicillin is predominantly active against gram-positive cocci and bacilli, as well as select spirochetes and gram-negative cocci like Neisseria, classifying it as a classic narrow-spectrum beta‑lactam.

Correct answer: B. Superinfection

Broad-spectrum antibiotics often destroy beneficial endogenous microbial flora, allowing opportunistic pathogens like Candida albicans or Clostridioides difficile to proliferate, causing a superinfection.

Correct answer: A. Aminoglycosides

Aminoglycosides (such as gentamicin and amikacin) target aerobic gram-negative bacilli and require oxygen-dependent active transport to enter bacterial cells, limiting their action against anaerobes.

Correct answer: C. It ensures coverage of likely pathogens while minimizing toxicity and resistance pressure

Empirical therapy relies on spectrum knowledge to cover potential pathogens before culture results return, balancing clinical efficacy with antimicrobial stewardship principles.

Correct answer: C. Antipseudomonal penicillins (e.g., Piperacillin)

Antipseudomonal penicillins like piperacillin have an extended spectrum that includes Pseudomonas aeruginosa and many enteric gram-negative rods, making them broader in scope than aminopenicillins.

Correct answer: B. Aminoglycosides

Aminoglycosides such as gentamicin and tobramycin primarily target aerobic Gram‑negative bacilli. Their uptake requires active transport dependent on oxygen, making them ineffective against obligate anaerobes.

Correct answer: B. Gram-positive organisms

Vancomycin is a glycopeptide that is active almost exclusively against Gram‑positive bacteria, including MRSA. Its large molecular size prevents penetration of the outer membrane of Gram‑negative organisms.

Correct answer: B. Destruction of normal resident microflora leading to superinfections

Broad‑spectrum antibiotics eliminate large portions of normal protective host flora, removing competitive inhibition and allowing opportunistic pathogens such as Clostridioides difficile or Candida species to proliferate, causing superinfections.

Correct answer: C. Carbapenems

Carbapenems, such as meropenem and imipenem, exhibit an extremely broad spectrum that includes aerobic Gram‑positive, Gram‑negative, and anaerobic pathogens like Bacteroides fragilis.

Correct answer: B. It minimizes the emergence of resistant bacterial strains and limits collateral damage to normal flora

Narrowing therapy based on culture results (de‑escalation) curtails selective pressure that drives antimicrobial resistance and preserves the patient’s endogenous protective microbiome.

Correct answer: B. Doxycycline

Doxycycline is a tetracycline derivative with a broad spectrum of activity covering Gram‑positive, Gram‑negative, atypical organisms (like Chlamydia and Mycoplasma), and certain protozoa.

Correct answer: B. Gram‑negative aerobic bacilli including Pseudomonas aeruginosa

Aztreonam is a monobactam that uniquely lacks activity against Gram‑positive and anaerobic bacteria, acting solely and effectively against aerobic Gram‑negative bacteria, including Pseudomonas aeruginosa.