A 12-year-old male presents with a 2-month history of a pruritic and scaly scalp. The patient was referred to dermatology by his pediatrician after failed empiric treatment with griseofulvin for presumptive tinea capitis. Since the initial examination performed by his pediatrician, the patient also developed erythematous, pruritic, scaly papules and plaques on bilateral arms, legs, and postauricular folds. His pediatrician prescribed low- to mid-potency topical corticosteroids, and he experienced mild improvement. Upon further questioning, the patient admitted to frequent self-scratching with subsequent development of lesions in the areas of irritation. He denied joint stiffness or pain. He denied a family history of psoriasis. With the exception of obesity and impaired fasting glucose, the patient was otherwise healthy and denied recent illness, sore throat, or sick contacts.

On physical examination, the scalp showed multiple areas of erythematous papules with overlying silvery scale coalescing to plaques on the right parietal-temporal scalp. The lesion spanned approximately 5 inches. No alopecia was appreciated. Bilateral posterior auricular folds had approximately 1-inch area of erythema with a thick white scale. The back and bilateral lower extremities had multiple areas of indurated pink plaques with loose micaceous scale. The left inguinal crease had a well-defined bright red erythematous patch without an overlying scale. Nail pitting was evident on the right second digit nail plate. There was no evidence of dactylitis or joint inflammation. Approximately 4% of the body surface area was affected.

Based on the clinical case description, what is the most likely diagnosis?