An active intravenous drug abuser presents to the emergency depament with fever of 5 days duration, a cough which is occasionally productive of blood, and pleuritic chest pain. Petechiae are present in his mouth and conjunctiva, and small linear hemorrhages are visible under his fingernails. Which of the following test results would MOST likely confirm the identity of the causative agent?

**Core Concept**
The patient's symptoms are suggestive of a severe infectious disease, likely endocarditis or another systemic infection, caused by a bacterium that is commonly associated with intravenous drug use (IVDU). The presence of petechiae, conjunctival hemorrhages, and small linear hemorrhages (splinter hemorrhages) under the fingernails are particularly suggestive of a bacterial infection that has disseminated to the skin and other organs.

**Why the Correct Answer is Right**
The patient's symptoms are highly suggestive of infective endocarditis caused by _Staphylococcus aureus_. This bacterium is a common cause of endocarditis in IVDU patients and can lead to embolic phenomena, including skin manifestations. The presence of a cough with occasional hemoptysis suggests pulmonary involvement. The correct test result would be one that directly targets _Staphylococcus aureus_.

**Why Each Wrong Option is Incorrect**
**Option A:** A positive result on a rapid plasma reagin (RPR) test would suggest syphilis, which can cause a wide range of symptoms including fever, cough, and skin manifestations. However, the presence of petechiae and splinter hemorrhages is not typical of syphilis.

**Option B:** A positive result on a hepatitis B surface antigen (HBsAg) test would indicate acute or chronic hepatitis B infection, which can cause fever, malaise, and jaundice. However, the patient's symptoms do not suggest hepatitis B infection.

**Option C:** A positive result on a human immunodeficiency virus (HIV) antibody test would indicate HIV infection, which can increase the risk of opportunistic infections. However, the patient's symptoms are more suggestive of a bacterial infection rather than an opportunistic infection.

**Option D:** A positive result on a _Candida_ antigen test would indicate candidemia or invasive candidiasis, which can cause fever, sepsis, and skin manifestations. However, the patient's symptoms are more suggestive of a bacterial infection rather than a fungal infection.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with fever, cough, and skin manifestations, especially in the context of IVDU, consider the possibility of infective endocarditis caused by _Staphylococcus aureus_.

**Correct Answer:** C. A positive result on a _Staphylococcus aureus_ antigen test or a positive blood culture for _Staphylococcus aureus_ would confirm the identity of the causative agent.

✓ Correct Answer: D. Blood culture of a coagulase-positive, catalase positive, gram-positive coccus