Which virus is likely responsible for a 4-year-old's stomatitis and vesicular rash on hands and feet?

The presence of stomatitis and a vesicular rash on the hands and feet of a 4-year-old is highly suggestive of hand, foot, and mouth disease, which is commonly caused by a Coxsackievirus, particularly Coxsackievirus A16. This viral infection is prevalent in young children and is characterized by fever, sore throat, and the appearance of vesicular lesions in the mouth and on the palms and soles. The typical presentation aligns with the symptoms observed in this age group.

Adenovirus is known to cause a variety of illnesses, including respiratory symptoms and conjunctivitis, but it is not typically associated with the specific pattern of symptoms presented here. Syphilis can cause a variety of skin manifestations but is unlikely to present as a vesicular rash specifically associated with stomatitis in a young child. Varicella, or chickenpox, is characterized by pruritic vesicular lesions that appear in a different distribution and generally begins with systemic symptoms and a rash that progresses throughout the body, rather than being limited to the hands and feet along with oral lesions.

Thus, the symptoms presented are consistent with a Coxsackievirus infection, making it the most appropriate choice for the clinical scenario described.