A 4-year-old white male is brought to your office in late August. His mother tells you that over the past few days he has developed a rash on his hands and sores in his mouth. On examination you note a vesicular exanthem on his hands, with lesions ranging from 3 to 6 mm in diameter. The oral lesions are shallow, whitish, 4- to 8-mm ulcerations distributed randomly over the hard palate, buccal mucosa, gingiva, tongue, lips, and pharynx. Except for a temperature of 37.4°C (99.3°F), the remainder of the examination is normal. The most likely diagnosis is