A 9-year-old boy is brought to the emergency department because of a severe sore throat with fever and dysphagia. On physical examination, there are grayish-white papulovesicular lesions on an erythematous base that ulcerate. They are located on the soft palate, anterior pillars of the tonsils and uvula. There are no lesions on the gingiva, tongue, or lips. A clinical diagnosis of herpangina is made. Which of the following is the most likely causative organism?