A 10-month-old infant presents with a rash, runny nose, cough, and white papules on the buccal mucosa. What is the likely diagnosis?

The clinical presentation of the infant strongly points towards a diagnosis of measles, particularly due to the classic signs associated with it. The combination of a rash, respiratory symptoms (runny nose and cough), and the presence of white papules on the buccal mucosa, known as Koplik spots, are hallmark signs of measles.

Measles typically begins with a prodromal phase characterized by symptoms such as high fever, cough, coryza (runny nose), and conjunctivitis, followed by the development of a characteristic maculopapular rash that usually appears a few days later. The white lesions seen on the buccal mucosa are indicative of Koplik spots, which are pathognomonic for measles.

While rubella, scarlet fever, and chickenpox can present with rashes and respiratory symptoms, they do not typically include Koplik spots or the exact arrangement and characteristics of the rash seen in measles. The other conditions have distinct clinical features that differentiate them from measles. Thus, given the combination of symptoms and the specific oral findings, the most likely diagnosis for this infant is measles.