Which disorder most commonly mimics a viral morbilliform eruption?

Drug eruptions (morbilliform drug eruptions) are often clinically indistinguishable from viral morbilliform eruptions. Less commonly, urticaria, arthropod reactions, early guttate psoriasis, and early pityriasis rosea can also be morbilliform.

Lerch M, Pichler WJ: The immunological and clinical spectrum of delayed drug-induced exanthems, Curr Opin Allergy Clin Immunol 4:411–419, 2004