A wound swab reveals gram-positive cocci in clusters, catalase-positive, sensitive to Cefoxitin and Oxacillin. What is the drug of choice?

1. A

   A. Ceftriaxone

2. B

   B. Vancomycin

3. C

   C. Cloxacillin

4. D

   D. Amoxicillin/Clavulanate

Appeared in: INI-CET EXAM -2026

Explanation

• The lab report describes gram-positive cocci in clusters that are catalase-positive, which is characteristic of Staphylococcus aureus.
• Crucially, the bacteria are sensitive to Cefoxitin and Oxacillin, identifying them as Methicillin-Sensitive S. aureus (MSSA).
• Cloxacillin is a penicillinase-resistant penicillin, which is the first-line drug of choice for treating MSSA infections.
• It is preferred over broader-spectrum antibiotics to minimize the risk of resistance and side effects.

Why Other Options Were Wrong

• Option A: Ceftriaxone is a third-generation cephalosporin. While it has broad-spectrum activity, it is less effective against MSSA compared to specific anti-staphylococcal penicillins like Cloxacillin.
• Option B: Vancomycin is the primary antibiotic for treating Methicillin-Resistant Staphylococcus aureus (MRSA). Using it for MSSA is inappropriate because Cloxacillin is more effective, and overuse of Vancomycin drives the development of vancomycin-resistant organisms.
• Option D: Amoxicillin/Clavulanate is a broad-spectrum antibiotic combination. While it has activity against MSSA, a narrow-spectrum agent like Cloxacillin is preferred to reduce the selection pressure for antibiotic resistance and minimize disruption to the patient's normal flora.

Related Visual

Clinical Relevance

• Nursing practice connection: Knowing Antibiotic selection for Methicillin-Sensitive Staphylococcus aureus (MSSA) based on laboratory susceptibility results helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
• Correctly interpreting antibiotic susceptibility reports is a fundamental nursing responsibility to ensure patient safety and effective treatment.
• This case highlights the principle of antibiotic stewardship: using the narrowest-spectrum, most effective agent to treat an infection. This prevents the development of multidrug-resistant organisms like MRSA and VRE.
• A nurse administering antibiotics must be aware of the differences between MSSA and MRSA to anticipate treatment plans, educate patients, and recognize potential treatment failures.

How to Approach the Question

• First, analyze the microbiology results provided in the question stem step-by-step.
• Identify the organism type from the Gram stain and morphology: 'gram-positive cocci in clusters' points towards Staphylococcus.
• Use the biochemical test result: 'catalase-positive' confirms it is Staphylococcus.
• Focus on the antibiotic susceptibility results: 'sensitive to Cefoxitin and Oxacillin' is the most critical piece of information. This tells you it is NOT resistant, i.e., it is Methicillin-Sensitive S. aureus (MSSA).
• Finally, recall or determine the first-line drug of choice for MSSA from the given options. Eliminate drugs used for MRSA (Vancomycin) or those that are broader-spectrum than necessary (Ceftriaxone, Amoxicillin/Clavulanate).

Concept Tested & Keywords

• Concept Tested: Antibiotic selection for Methicillin-Sensitive Staphylococcus aureus (MSSA) based on laboratory susceptibility results.
• Stem keywords: gram-positive cocci, catalase-positive, Cefoxitin, Oxacillin, sensitive, drug of choice
• Lead-in keywords: What is
• Clinical cues: The key clinical cue is sensitivity to both Cefoxitin and Oxacillin, which definitively identifies the pathogen as Methicillin-Sensitive Staphylococcus aureus (MSSA).

Question ID

QPFB34X1S4sKVY5tKsDif5

Copy ID

Reference Book

E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 1702-1704

E6 Medicine Harrison 22e Part 1 p. 1244-1246