Koplik's spots are associated with which of the following conditions?

Koplik's spots are characteristic lesions that appear in the oral cavity and are specifically associated with measles (rubeola). These spots are small, bluish-white lesions with a red halo that appear on the buccal mucosa, typically two to three days before the rash associated with measles develops. They serve as an important clinical sign that helps healthcare providers identify and diagnose measles early in the course of the illness.

In contrast, the other conditions listed do not present with Koplik's spots. Poxvirus infections, such as those caused by variola or vaccinia, are associated with different types of skin lesions, typically vesicular or pustular lesions on the skin. Kawasaki's disease is a vasculitis affecting children and is characterized by high fever, rash, and conjunctivitis but does not include Koplik's spots as a symptom. Reye's syndrome is linked to a viral infection and aspirin use in children, presenting primarily with encephalopathy and liver dysfunction rather than oral lesions like Koplik's spots. Thus, the association of these spots specifically with measles makes it clear that option B is the correct choice.