What is the most likely diagnosis for a 3-year-old with conjunctival injection, fever, and rash?

The most likely diagnosis for a 3-year-old with conjunctival injection, fever, and rash is Kawasaki disease. This condition is characterized by a range of symptoms, including fever lasting more than five days, conjunctival injection (red eyes), a polymorphous rash, and often also includes symptoms such as changes in the lips and oral cavity, cervical lymphadenopathy, and hand and foot changes.

Kawasaki disease is primarily an acute vasculitis that can lead to serious complications if not recognized and treated promptly, particularly affecting the coronary arteries. The presence of conjunctival injection is a common finding in Kawasaki disease, differentiating it from some other conditions that may present with similar symptoms.

In contrast, while other options such as scarlet fever, hand-foot-and-mouth disease, and measles may present with fever and rash, they typically have additional specific features that would make them less likely in this scenario. For instance, scarlet fever often presents with a sandpaper-like rash and throat symptoms (like a sore throat), while hand-foot-and-mouth disease primarily features oral lesions and rash on the hands and feet. Measles is characterized by a specific type of rash and a history of exposure, as well as Koplik spots in the mouth.