Keratosis Pilaris: A common skin condition characterized by small, rough, sandpaper-like bumps on the skin, often appearing as a dark red papular rash, particularly on the back and buttocks. It's more pronounced in dry skin conditions.

Eczema/Dermatitis: Can present with a dark red papular rash, especially if it's an allergic or irritant contact dermatitis. The location on the back and buttock could be due to allergens or irritants coming into contact with these areas.
Folliculitis: An inflammation of the hair follicles, which can appear as small, dark red bumps, particularly in areas with dense hair follicles like the back and buttocks.
Pityriasis Rosea: Although it typically starts with a herald patch, it can progress to a widespread rash that might appear on the back and buttocks, characterized by small, oval, dark red papules.

Meningococcemia: A severe infection caused by Neisseria meningitidis, which can present with a petechial or purpuric rash that might initially appear as dark red papules. It's critical to identify this condition early due to its high mortality rate if left untreated.
Rocky Mountain Spotted Fever (RMSF): A tick-borne disease that can cause a rash, which often appears as small, flat, dark red spots. Although it typically starts on the wrists and ankles, it can spread to other areas, including the back and buttocks.

Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous, well-demarcated papules and plaques, which could fit the description of a dark red papular rash.
Erythema Multiforme: An immune-mediated disorder that can present with a variety of skin manifestations, including target lesions and papules, which might appear as a dark red rash on the back and buttocks.