A 57-year-old man arrives at the emergency room complaining of weakness, fatigue, and intermittent fever that has recurred for several weeks. The patient had a cardiac valvular prosthesis implanted 5 years ago. Blood cultures grew ceain gram-positive cocci on which catalase and coagulase test were done. The gram-positive organisms failed to ferment mannitol, and their growth was inhibited by novobiocin. CATALASE TEST SLIDE COAGULASE TEST Lesions seen on skin What is the most likely infectious agent?

**Core Concept**
The patient's symptoms, along with the results of blood cultures and biochemical tests, point towards a bacteremia caused by a coagulase-negative staphylococcus. These organisms are common contaminants in medical devices and prosthetic material, and their growth can be inhibited by novobiocin.

**Why the Correct Answer is Right**
The catalase test distinguishes staphylococci from streptococci, as staphylococci produce catalase, an enzyme that breaks down hydrogen peroxide. The coagulase test differentiates between coagulase-positive (e.g., Staphylococcus aureus) and coagulase-negative staphylococci. Staphylococcus epidermidis fails to ferment mannitol, which is a key differentiator from other coagulase-negative staphylococci. Additionally, its growth is inhibited by novobiocin, a characteristic used to identify S. epidermidis. This organism is a common cause of bacteremia in patients with prosthetic material, as it can colonize and infect the device.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a coagulase-positive organism that ferments mannitol, which does not match the biochemical profile of the organism described in the question.

**Option C:** Staphylococcus saprophyticus is a coagulase-negative organism that ferments mannitol, which does not match the biochemical profile of the organism described in the question.

**Option D:** Streptococcus viridans is a catalase-negative organism that ferments mannitol, which does not match the biochemical profile of the organism described in the question.

**Clinical Pearl / High-Yield Fact**
Staphylococcus epidermidis is a common cause of bacteremia in patients with prosthetic material, and its growth can be inhibited by novobiocin. This characteristic can be used to identify S. epidermidis in the laboratory.

**✓ Correct Answer: B. Staphylococcus epidermidis**

✓ Correct Answer: B. Staphylococcus epidermidis