What is the most likely diagnosis for a 12-year-old girl with sore mouth and numerous yellow-grey lesions?

The presence of multiple yellow-grey lesions in the mouth, combined with the symptom of a sore mouth, strongly suggests herpetic gingivostomatitis. This condition is commonly caused by the herpes simplex virus, particularly in children and adolescents. The lesions typically present as vesicular sores that can appear on the gums, tongue, and inner cheeks, often accompanied by pain and difficulty eating or drinking.

In this case, the age of the patient—12 years old—is also significant as this is a common age for herpetic gingivostomatitis to manifest for the first time following primary infection with the herpes simplex virus. Other symptoms that may accompany herpetic gingivostomatitis include fever and malaise, further reinforcing the diagnosis.

Other conditions mentioned have different presentations. Measles typically presents with a distinct rash and Koplik spots inside the mouth rather than numerous yellow-grey lesions. Erythema multiforme is associated with target lesions on the skin and can involve mucosal surfaces but does not commonly present as yellow-grey mouth lesions. Stevens-Johnson syndrome is a severe condition that results in widespread skin and mucosal involvement but usually arises with systemic symptoms and a more severe clinical profile than described in the question. Thus, her