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 laboratory report describes Methicillin-Sensitive Staphylococcus aureus (MSSA). The description 'gram-positive cocci in clusters' is characteristic of Staphylococcus, and being 'catalase-positive' confirms it.
• The key differentiator is the sensitivity to Cefoxitin and Oxacillin. This confirms the bacteria is susceptible to penicillinase-resistant penicillins.
• Cloxacillin is a penicillinase-resistant penicillin, making it a first-line drug of choice for treating MSSA infections.
• Using a narrow-spectrum agent like Cloxacillin is preferred for MSSA to effectively treat the infection while minimizing the risk of promoting antibiotic resistance and disrupting the patient's normal flora.

Why Other Options Were Wrong

• Option A: Ceftriaxone is a third-generation cephalosporin with a broad spectrum of activity. While it may have some effect, it is not the targeted, first-line therapy for a confirmed MSSA infection. Using such a broad agent when a narrower one is effective is poor antibiotic stewardship.
• Option B: Vancomycin is reserved for treating infections caused by Methicillin-Resistant Staphylococcus aureus (MRSA). Using it for MSSA is inappropriate, contributes to the risk of developing vancomycin-resistant organisms, and is less effective than beta-lactam antibiotics for MSSA.
• Option D: Amoxicillin combined with Clavulanate (a beta-lactamase inhibitor) is effective against MSSA. However, it is a broader-spectrum antibiotic than Cloxacillin. A more targeted, narrow-spectrum agent like Cloxacillin is preferred as the first-line treatment to reduce side effects and resistance pressure.

Related Visual

Clinical Relevance

• Nursing practice connection: Knowing The primary concept tested is the identification and first-line antimicrobial treatment 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.
• Nurses are responsible for administering antibiotics and must understand the rationale for choosing a specific drug. Knowing the difference between MSSA and MRSA is fundamental to safe practice and antibiotic stewardship.
• A key nursing role is to review lab results and question antibiotic orders that seem inconsistent. For example, a nurse should clarify an order for Vancomycin if the culture shows MSSA.
• Patient education is vital. Nurses should explain the importance of completing the full course of antibiotics to prevent the development of resistant bacteria.

How to Approach the Question

• First, break down the laboratory findings in the question stem. Note each piece of information: Gram stain, morphology, catalase test, and antibiotic sensitivities.
• Synthesize the initial findings: 'Gram-positive cocci in clusters' + 'catalase-positive' points directly to the Staphylococcus genus.
• Focus on the most critical information for drug selection: the sensitivity report. 'Sensitive to Cefoxitin and Oxacillin' is the defining characteristic of Methicillin-Sensitive S. aureus (MSSA).
• Recall the first-line treatment for MSSA. The standard of care is a penicillinase-resistant penicillin.
• Evaluate the given options. Identify which drug belongs to the class of penicillinase-resistant penicillins. Cloxacillin is the correct choice.
• Eliminate the other options based on their primary use: Vancomycin for MRSA, Ceftriaxone for broader gram-negative coverage, and Amoxicillin/Clavulanate as a broader-spectrum agent not typically used as a first line for confirmed MSSA.

Concept Tested & Keywords

• Concept Tested: The primary concept tested is the identification and first-line antimicrobial treatment for Methicillin-Sensitive Staphylococcus aureus (MSSA) based on laboratory susceptibility results.
• Stem keywords: wound swab, gram-positive cocci in clusters, catalase-positive, sensitive to Cefoxitin, sensitive to Oxacillin
• Lead-in keywords: drug of choice
• Clinical cues: The combination of Gram stain morphology (cocci in clusters) and a positive catalase test is a classic identifier for Staphylococcus species.
• Clinical cues: Sensitivity to both Cefoxitin and Oxacillin is the definitive laboratory marker that differentiates MSSA from the more resistant MRSA.

Question ID

Qy8V-06DKf65N9vdpe4WVE

Copy ID

Reference Book

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

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