An otherwise healthy patient who has just received a prosthetic aoic valve develops postoperative fever. Blood cultures are done and she is placed on broad-spectrum antibiotics. Two days later she is still febrile and clinically deteriorating. Which of the following organisms is the most probable etiologic agent?

The patient likely has a candidal infection of the prosthetic aoic valve. That is why she did not respond orably to antibacterial therapy, which is known to promote fungal infection. Note that Candida tends to colonize foreign bodies such as IV and Foley catheters, prosthetic valves, and ventricular shunts. Actinomyces israelii is known to cause cervicofacial infections in patients having undergone dental work. Histoplasma capsulatum causes histoplasmosis, a pulmonary infection common in the midwestern river valleys. Multi-organ involvement is usually seen only in the immunocompromised. Transmission of the organism occurs through the inhalation of airborne microconidia (infectious) spores. The organism is found in bird and bat droppings and in the soil. Nocardia asteroides is an actinomycete that causes a chronic lobar pneumonia that may metastasize to the brain. It is more common in the immunocompromised. It is found in soil and aquatic environments. Ref: Ray C.G., Ryan K.J. (2010). Chapter 45. Candida, Aspergillus, Pneumocystis,and Other Oppounistic Fungi. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.