Small grayish vesicles and punched-out ulcers in a child's posterior pharynx suggest which organism?

The presence of small grayish vesicles and punched-out ulcers in a child's posterior pharynx is characteristic of an infection caused by the Coxsackievirus. This virus is part of the enterovirus family and is known to cause herpangina, which often presents with lesions in the mouth and throat. The vesicles can quickly rupture, leading to the formation of painful ulcers that appear "punched-out."

Coxsackievirus infections are more common in young children and are typically associated with symptoms such as fever, sore throat, and difficulty swallowing, consistent with the findings in this scenario. Recognizing the specific lesions helps to differentiate Coxsackievirus from other pathogens.

In contrast, Epstein-Barr virus primarily causes infectious mononucleosis, characterized by enlarged lymph nodes and fatigue rather than small vesicles. Group C Streptococcus infections might lead to symptoms of pharyngitis but would not typically present with vesicular lesions. Gonorrhea is primarily associated with genital infections and would be less relevant in the context of a child's pharyngeal symptoms. Thus, the clinical presentation aligns most closely with an infection caused by the Coxsackievirus.