Pityriasis Rosea: This condition is characterized by the appearance of a herald patch followed by a widespread eruption of pink, scaly papules, typically on the torso and extremities. The age of onset and the description of the lesions make this a strong candidate.

Viral Exanthem: Various viral infections can cause diffuse pink papules, including enteroviruses, adenoviruses, and parvovirus B19. The presentation can vary but often includes fever and other systemic symptoms.
Atopic Dermatitis (Eczema): While atopic dermatitis typically presents with more itching and lichenification, in its early stages or in mild cases, it can manifest as diffuse pink papules.
Papular Urticaria: This condition is an allergic reaction to insect bites or other allergens, leading to itchy papules. It's more common in children but can occur at any age.

Secondary Syphilis: This condition can cause a wide range of skin manifestations, including diffuse pink papules. It's crucial to consider syphilis due to its serious complications if left untreated.
Lymphoma Cutis: Certain types of lymphoma can initially present with skin manifestations, including papules. Although rare, missing this diagnosis could have severe consequences.
Drug Eruption: Various medications can cause skin reactions, including diffuse pink papules. Given the potential for serious drug reactions, this should always be considered, especially if the patient has recently started new medications.

Pityriasis Lichenoides: This is a rare skin condition characterized by the appearance of small, scaling papules that can resemble pityriasis rosea but often has a more chronic course.
Keratosis Pilaris: While typically presenting with rough, dry skin and small bumps, in some cases, it can appear as pink papules, especially if inflamed.
Sarcoidosis: A systemic disease that can cause skin lesions, including papules, although this is less common. Sarcoidosis would typically be accompanied by other systemic symptoms.