**Question:** A 50-year-old man presented to his family doctor complaining of flu-like symptoms. The doctor gave him a shot of penicillin and sent him home. The man came back a week later and in fact, felt worse. The man was a rancher and raised a lot of cattle. He told the doctor that he had congenital heart disease and an attack of rheumatic fever when he was young. The man now complained of fever, night sweats, continual coughing, weight loss, and lethargy. The doctor had the man hospitalized because he now suspected subacute bacterial endocarditis (SBE). A culture of his blood did not show any bacterial growth, but a serum analysis for antibodies against a certain bacterium was positive. As a result of finding antibodies to this organism, treatment with doxycycline was begun and the patient improved and was released from the hospital. What was the name of the organism with which the man was infected?

A. Streptococcus pyogenes
B. Streptococcus pneumoniae
C. Staphylococcus aureus
D. Mycoplasma pneumoniae

**Correct Answer:** C. Staphylococcus aureus

**Core Concept:**
Subacute bacterial endocarditis (SBE) is a type of bacterial endocarditis, which is an infection of the heart valves and surrounding tissues. The most common cause of SBE is the bacterium Staphylococcus aureus. SBE can be classified into three types: acute, subacute, and chronic. In this case, the patient's symptoms and history suggest subacute bacterial endocarditis, and the absence of bacterial growth in blood culture indicates that the infection is likely caused by a bacterium that is difficult to culture, such as Staphylococcus aureus or Streptococcus pneumoniae.

**Why the Correct Answer is Staphylococcus aureus:**

The patient's history of rheumatic fever and congenital heart disease makes Staphylococcus aureus a likely cause for the subacute bacterial endocarditis. Although a blood culture showed no bacterial growth, the presence of antibodies against the bacterium in the patient's serum indicates an active infection.

**Why the Wrong Answers are Incorrect:**

A. Streptococcus pyogenes (Streptococcus pyogenes), also known as Group A Streptococcus (GAS), is the bacterium that causes rheumatic fever and is associated with rheumatic heart disease, similar to the patient's history. However, the patient's symptoms and blood culture results make Staphylococcus aureus a more likely cause of the infection.

B. Streptococcus pneumoniae (Streptococcus pneumoniae) is a common cause of pneumonia and meningitis but is not typically associated with congenital heart disease like the patient's history suggests. Additionally, the patient's symptoms and blood culture results make Staphylococcus aureus a more likely cause of the infection.

C. Mycoplasma pneumoniae is a bacterium that causes pneumonia and is usually cultured from the patient's respiratory secretions, not blood. The patient's history of congenital heart disease and the negative blood culture make Staphylococcus a