**Core Concept**
The patient's prosthetic aortic valve increases his risk for prosthetic valve endocarditis (PVE), which is a type of infective endocarditis that occurs on a prosthetic valve. The presence of a prosthetic valve alters the normal valve architecture, making it more susceptible to bacterial colonization and infection.

**Why the Correct Answer is Right**
The most common pathogens responsible for PVE are Staphylococcus epidermidis, a coagulase-negative Staphylococcus species that is part of the normal skin flora. S. epidermidis can colonize the prosthetic valve and cause infection due to its ability to form biofilms, which protect the bacteria from the host's immune system and antibiotic therapy. The patient's symptoms of fever, chills, and malaise, along with the presence of a prosthetic valve, make S. epidermidis a likely causative organism.

**Why Each Wrong Option is Incorrect**
**Option A:** Streptococcus viridans is a common cause of native valve endocarditis, but it is less commonly associated with prosthetic valve endocarditis.
**Option B:** Enterococcus faecalis can cause endocarditis, but it is not as commonly associated with prosthetic valve endocarditis as S. epidermidis.
**Option D:** Pseudomonas aeruginosa can cause endocarditis, but it is not as commonly associated with prosthetic valve endocarditis as S. epidermidis.

**Clinical Pearl / High-Yield Fact**
In patients with prosthetic valves, PVE should be suspected in the presence of fever, chills, and malaise, especially if the valve has been in place for more than 1 year. A high index of suspicion is necessary for early diagnosis and treatment.

**Correct Answer: D. Staphylococcus epidermidis.**