Pityriasis rosea: This is a common skin condition characterized by a papular rash, often starting with a herald patch, followed by a widespread rash on the trunk. The rash typically appears as oval-shaped, scaly patches that are oriented in a Christmas tree-like pattern.

Eczema (atopic dermatitis): Can present with papular lesions on the trunk, among other areas, and is often associated with itching and a personal or family history of atopy.
Contact dermatitis: An allergic or irritant reaction causing a papular rash on the trunk, depending on exposure to offending substances.
Folliculitis: Inflammation of the hair follicles, which can appear as small, itchy papules on the trunk, often due to bacterial or fungal infections.
Keratosis pilaris: A condition characterized by small, rough, sandpapery bumps on the skin, often on the trunk, due to keratin buildup in the hair follicles.

Syphilis (secondary): Can present with a diffuse, papular rash on the trunk, among other symptoms, and is critical to diagnose due to its serious complications if left untreated.
Lyme disease: May cause a rash (erythema migrans) that can appear as a papular lesion, although it's more commonly associated with a distinctive "bull's-eye" pattern.
Meningococcemia: A severe infection that can cause a petechial or papular rash, which is a medical emergency.

Papular urticaria: An itchy rash caused by an allergic reaction to insect bites or other stimuli, which can appear as papules on the trunk.
Mucha-Habermann disease (Pityriasis lichenoides et varioliformis acuta): A rare skin condition characterized by an acute onset of papular lesions that can evolve into vesicles or crusts, often on the trunk.
Sarcoidosis: A systemic disease that can cause skin lesions, including papules, on the trunk, among other symptoms like lung involvement and lymphadenopathy.