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The answer is b, Parvovirus B19.

Fifth disease (erythema infectiosum), long recognized as a benign mild exanthem of school-age children, is now known to be caused by human parvovirus B19. In the compromised patient, the parvovirus can cause serious anemia by infecting red-cell precursors and causing them to lyse. Patients with hemolytic conditions, such as sickle cell anemia, thus develop a transient aplastic crisis. A poorly functioning bone marrow (for a week or more) in a patient with a reduced red-cell life span (about 30 days) can result in profound anemia.

Other problems can result in patients infected with parvovirus B19. In patients with immunodeficiency, the B19 infection can be persistent and lead to life-threatening chronic anemia. Infection in a pregnant woman can result in severe anemia in the infected fetus, with secondary hydrops fetalis and death. Roseola is now thought to be caused most often by the human herpesvirus 6. Coxsackie A16 virus causes hand-foot-and-mouth disease. Echo-11 virus frequently causes viral meningitis, and cytomegalovirus causes a congenital infection.