A 6-year-old girl has a blotchy, reddish-brown rash on her face, trunk, and proximal extremities that developed over the course of 3 days. On physical examination, she has 0.2-cm to 0.5-cm ulcerated lesions on the oral cavity mucosa and generalized tender lymphadenopathy. A cough with minimal sputum production becomes progressively worse over the next 3 days. Which of the following viruses is most likely to produce these findings?

The rash and the Koplik spots on the buccal mucosa are characteristic findings in measles (rubeola), a childhood infection. It occurs only sporadically when immunizations have been administered to a large part of the population. The severity of the illness varies, and measles pneumonia may complicate the course of the disease, which in some cases can be life-threatening. Mononucleosis, which results from Epstein-Barr (EBV) virus infection, is more likely to occur in adolescence. Mumps produces parotitis and orchitis. Varicella-zoster virus infections in children manifest as chicken-pox. Rubella, also called German measles, is a much milder infection than rubeola.