A 3-year-old child presents with fever and vesicular lesions on the hands, feet, and inside the mouth. What is the causative agent?

Hand, Foot, and Mouth Disease (HFMD) is caused primarily by Coxsackievirus A16 and occasionally by Enterovirus 71. It is a highly contagious viral illness most common in children under 5 years old. Clinical features include fever followed by painful vesicular lesions on the hands, feet, buttocks, and oral mucosa (particularly the tongue, buccal mucosa, and soft palate). The oral lesions rupture to form shallow ulcers. The combination of the specific distribution (hands, feet, and mouth) distinguishes HFMD from primary herpetic gingivostomatitis (which is primarily intraoral) and chickenpox (which has a centripetal distribution with no predilection for hands and feet).