What's the most likely diagnosis in this dog?
A 5-year-old spayed female mixed-breed dog is presented for evaluation of crusted and eroded plaques and nodules on the dorsal aspect of its muzzle. The lesions appeared quickly (less than 24 hours) and have been present for about one week. They are moderately pruritic and painful. The dog is bright, alert, and responsive, and no other skin lesions are present.
What is the most likely diagnosis?