Final answer:
The most likely diagnosis is infective endocarditis, an infection of the inner lining of the heart and heart valves.
Step-by-step explanation:
The most likely diagnosis in an IV drug user with a new heart murmur and fever is infective endocarditis. Infective endocarditis is an infection of the inner lining of the heart and heart valves. It often occurs in individuals with predisposing factors such as IV drug use, as bacteria can enter the bloodstream through injection sites and infect the heart.
In this case, the presence of a new heart murmur suggests valve damage or inflammation, which is a common finding in infective endocarditis. The fever further supports the diagnosis as it is a common symptom of infection.
Additional diagnostic tools such as blood cultures and echocardiograms can confirm the diagnosis. Treatment typically involves high doses of intravenous antibiotics.