Question:
A patient presents with septic arthritis. Culture grows a Gram-positive coccus that is catalase positive and cefoxitin sensitive. What is the drug of choice?
A patient presents with septic arthritis. Culture grows a Gram-positive coccus that is catalase positive and cefoxitin sensitive. What is the drug of choice?
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Cefoxitin sensitive = MSSA, so use a penicillinase-resistant penicillin, not vancomycin.
Updated On: Jun 25, 2026
- Cloxacillin
- Penicillin
- Vancomycin
- Linezolid
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The Correct Option is A
Solution and Explanation
Step 1: Identify the organism.
A Gram-positive coccus that is catalase positive is a Staphylococcus (streptococci are catalase negative). In septic arthritis, the commonest cause is Staphylococcus aureus.
Step 2: Interpret the cefoxitin result.
Cefoxitin is the surrogate disc used to detect the mecA-mediated methicillin resistance (MRSA). Cefoxitin sensitive means the isolate is methicillin-sensitive S. aureus (MSSA), not MRSA.
Step 3: Choose the drug.
For MSSA, the drugs of choice are the anti-staphylococcal penicillinase-resistant penicillins - cloxacillin / flucloxacillin / nafcillin / oxacillin. These are more effective than vancomycin against MSSA.
Step 4: Eliminate the distractors.
Plain penicillin G is destroyed by staphylococcal beta-lactamase, so >90% of S. aureus are resistant - not reliable. Vancomycin and linezolid are reserved for MRSA (cefoxitin-resistant) isolates and are actually inferior to cloxacillin for MSSA.
Key fact: Cefoxitin-sensitive (MSSA) staphylococcal infection to treat with an anti-staphylococcal penicillin such as cloxacillin.
A Gram-positive coccus that is catalase positive is a Staphylococcus (streptococci are catalase negative). In septic arthritis, the commonest cause is Staphylococcus aureus.
Step 2: Interpret the cefoxitin result.
Cefoxitin is the surrogate disc used to detect the mecA-mediated methicillin resistance (MRSA). Cefoxitin sensitive means the isolate is methicillin-sensitive S. aureus (MSSA), not MRSA.
Step 3: Choose the drug.
For MSSA, the drugs of choice are the anti-staphylococcal penicillinase-resistant penicillins - cloxacillin / flucloxacillin / nafcillin / oxacillin. These are more effective than vancomycin against MSSA.
Step 4: Eliminate the distractors.
Plain penicillin G is destroyed by staphylococcal beta-lactamase, so >90% of S. aureus are resistant - not reliable. Vancomycin and linezolid are reserved for MRSA (cefoxitin-resistant) isolates and are actually inferior to cloxacillin for MSSA.
Key fact: Cefoxitin-sensitive (MSSA) staphylococcal infection to treat with an anti-staphylococcal penicillin such as cloxacillin.
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