Viral Exanthem: This is often the most common cause of a morbilliform rash, which can be due to various viral infections such as measles, rubella, or roseola, although these are less common in adults. The rash typically starts on the face and spreads downward, including the neck.

Drug Reaction: Many medications can cause a morbilliform drug eruption, which is a common type of drug rash. This can occur with antibiotics, anticonvulsants, and NSAIDs, among others. The rash usually appears within a week or two of starting the new medication.
Allergic Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen, leading to a rash. Common allergens include nickel, fragrances, and latex. The rash is typically confined to the area of contact.
Atopic Dermatitis (Eczema): While more common in children, atopic dermatitis can persist into adulthood. It often involves the neck and can present with a morbilliform appearance, especially if exacerbated by allergens or irritants.

Secondary Syphilis: This condition can cause a wide range of skin manifestations, including a morbilliform rash. It's crucial to consider syphilis due to its potential for serious complications if left untreated.
Lymphoma: Certain types of lymphoma, like cutaneous T-cell lymphoma, can initially present with skin manifestations that might resemble a morbilliform rash. Early diagnosis is critical for effective treatment.
Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition can sometimes present with a rash, particularly if the patient is taking ampicillin.

Pityriasis Rosea: Although not typically morbilliform, pityriasis rosea can sometimes be considered in the differential for a rash on the neck, especially if the patient has recently experienced an upper respiratory infection.
Kawasaki Disease: While predominantly seen in children, Kawasaki disease can very rarely occur in adults and presents with fever, rash, and other systemic symptoms.
Erythema Multiforme: This is an immune-mediated condition that can be triggered by infections or medications, characterized by target lesions, but can sometimes appear morbilliform, especially in its early stages.