Final answer:
The most common cause of dilated cardiomyopathy in relatively young adults is genetic or familial cardiomyopathy.
Step-by-step explanation:
Dilated cardiomyopathy (DCM) is a condition characterized by the enlargement and weakening of the heart's left ventricle. In relatively young adults, the most common cause of DCM is genetic or familial cardiomyopathy. This means that there is a hereditary component to the condition, and it may run in families. Mutations in certain genes, such as the genes encoding sarcomeric proteins, can lead to the development of DCM. Other causes of DCM in young adults can include viral infections, alcohol or drug abuse, and certain autoimmune diseases.