Microbiology: USMLE Q No. 15

[Cardiac Arrest]

A 68-year-old woman develops persistent pain in her right hip following a total cox arthroplasty. Radiography of the joint reveals loosening of the prosthesis. On microscopic examination of synovial aspirate, the isolate does not ferment mannitol and is catalase positive, coagulase negative and novobiocin sensitive. Which of the following is the most likely causative organism?


A. Neisseria gonorrhoeae
B. Staphylococcus aureus
C. Staphylococcus epidermidis
D. Staphylococcus saprophyticus
E. Yersinia enterocolitica

[Dr_makkawi]

C. Staphylococcus epidermidis :122:

[Cardiac Arrest]

Option C (Staphylococcus epidermidis) is correct. S. epidermidis has an affinity toward prosthetic devices and is frequently recovered from the synovial fluid of patients who have undergone arthroplasty.

Option A (Neisseria gonorrhoeae) is incorrect. Gonococci do not grow on blood agar and appear as Gram-negative, kidney bean-shaped diplococci in smears.

Option B (Staphylococcus aureus) is incorrect. S. aureus produces creamy colonies on blood agar that are usually surrounded by beta hemolysis. The key diagnostic criterion that differentiates S. aureus from other staphylococci is a positive coagulase test.

Option D (Staphylococcus saprophyticus) is incorrect. S. saprophyticus is novobiocin resistant.

Option E (Yersinia enterocolitica) is incorrect. Like most members of the Enterobacteriaceae family, Y. enterocolitica produces large, mucoid colonies on blood agar. It is a Gram-negative, rod-shaped bacterium.

[f_ameer376]

thanx for the ques