Pityriasis Rosea: This is the most likely diagnosis due to the characteristic presentation of a herald patch followed by a secondary eruption of oval-shaped, scaly patches in a Christmas tree distribution. The clinical appearance and the typical age of onset (10-35 years) support this diagnosis.

Contact Dermatitis: Given the presence of skin lesions, contact dermatitis is a plausible diagnosis, especially if the patient has been exposed to allergens or irritants. The distribution and morphology of the lesions can help differentiate it from pityriasis rosea.
Nummular Dermatitis: This condition presents with coin-shaped patches of eczema and can be considered in the differential, especially if the lesions are more circular and not as typical for pityriasis rosea.
Seborrheic Dermatitis: Although it typically affects the scalp, face, and torso, seborrheic dermatitis can sometimes present with a more widespread distribution, making it a consideration in the differential diagnosis.

Syphilis (Secondary): The rash of secondary syphilis can be quite varied and may mimic pityriasis rosea or contact dermatitis. It's crucial to consider syphilis due to its significant implications for treatment and public health.
Lymphoma (Cutaneous T-Cell Lymphoma): Although rare, cutaneous lymphomas can present with skin lesions that resemble those of pityriasis rosea or dermatitis. Missing this diagnosis could have severe consequences.

Pityriasis Lichenoides: A rare skin condition that can present with a variety of lesions, including papules, plaques, and nodules, making it a less common but possible differential diagnosis.
Guttate Psoriasis: While more commonly seen in children, guttate psoriasis can occur in adults and presents with small, droplike lesions, which might be considered in the differential, especially if other psoriatic features are present.