What is the most likely diagnosis for a child presenting with posterior oropharyngeal vesicles and fever?

The presentation of posterior oropharyngeal vesicles accompanied by fever is highly indicative of herpangina. This condition is typically caused by Coxsackievirus, a member of the enterovirus family, and is characterized by the sudden onset of fever, sore throat, and the appearance of vesicular lesions on the soft palate or tonsils. The vesicles can quickly ulcerate, leading to painful sores that contribute to the child’s discomfort.

While hand, foot, and mouth disease also involves vesicular lesions, it primarily affects the anterior oropharynx, hands, and feet rather than the posterior oropharynx, making it less likely in this scenario. Strep throat, caused by group A Streptococcus, typically presents with pharyngeal inflammation, but it does not usually result in vesicular lesions. Herpes simplex virus infection can present with similar symptoms, but it is more likely to cause lesions on the lips or around the mouth, rather than specifically in the posterior oropharynx as the first sign of illness.

Thus, based on the combination of posterior oropharyngeal vesicles and fever, herpangina stands out as the most plausible diagnosis.