A 10-month-old child is fussy, refuses to eat. Disease onset was 2 days ago. The child is been treated by a pediatrician for pneumonia, receives antibiotics and sulfanilamides. Objectively: the oral mucosa is hyperemic, swollen; there is whitish coating on the mucosa of the cheeks, lips, soft and hard palate; coating removal can cause erosions. Submandibular lymph nodes are enlarged. What is the most likely diagnosis?