A young adult presenting with swelling of the parotid glands is most likely to have which of the following diseases?

The presence of swelling of the parotid glands is a hallmark sign of mumps, a viral infection caused by the mumps virus. This condition is characterized by bilateral parotitis, which is the swelling of the salivary glands located near the jawline and ears. Mumps can lead to complications, such as orchitis in males and viral meningitis, but the key clinical feature to note is the distinct swelling of the parotid glands.

In contrast, measles, rubella, and congenital rubella do not primarily present with parotid gland swelling. Measles typically presents with symptoms like high fever, cough, and the characteristic measles rash, while rubella is usually associated with a lighter rash and mild symptoms. Congenital rubella syndrome affects infants whose mothers were infected during pregnancy, leading to a range of birth defects, but again does not involve parotid gland swelling like mumps does. Therefore, the identification of parotid gland swelling directs towards mumps as the most likely diagnosis.