Hand-Foot-Mouth Disease — Coxsackie Vesicles

A 4-year-old child presents with a 2-day history of fever, sore throat, and poor feeding. On examination, the structure marked **C** in the diagram shows painful ulcers with red halos on the soft palate, tonsillar pillars, and buccal mucosa. The child also has oval grey-white vesicles on the dorsal and lateral aspects of hands and feet. Which of the following is the most likely causative agent?