What disease primarily affects the heart muscle?

Myocarditis specifically refers to the inflammation of the heart muscle, or myocardium. This condition can arise from various causes, including viral infections, autoimmune diseases, or exposure to certain toxins. The inflammation can lead to conditions that disturb the heart’s ability to pump blood effectively and can result in heart failure or other serious complications if left untreated. This makes it the most appropriate choice when identifying a disease that primarily affects the heart muscle.

In contrast, endocarditis is an infection of the inner lining of the heart, particularly affecting the heart valves rather than the heart muscle itself. Melanoma is a type of skin cancer that originates in pigment-producing cells and doesn't affect the heart muscle. Fibrosarcoma is a malignant tumor composed of fibrous connective tissue and is not specific to the heart, as it can occur in various parts of the body. Therefore, myocarditis is the disease that directly impacts the heart muscle, distinguishing it clearly from the others.