In IV drug user with a new murmur and fever, what is the most likely diagnosis?

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In IV drug users, fever with a new heart murmur strongly points to infective endocarditis. The act of injecting drugs can introduce bacteria into the bloodstream, which then seed heart valves and form vegetations. These vegetations damage the valve, causing regurgitation that produces a new or changing murmur, and the ongoing infection causes fever. The tricuspid valve is the most commonly affected valve in this setting, though any valve can be involved. By contrast, pericarditis usually presents with chest pain that worsens with inspiration and a friction rub, myocardial infarction with chest pain plus characteristic ECG changes and elevated troponin, and aortic dissection with sudden, severe chest or back pain and often blood pressure differences. So the combination of IV drug use, fever, and a new murmur makes infective endocarditis the best explanation. Diagnostic steps include obtaining blood cultures and performing echocardiography to detect vegetations.