This patient's rash is consistent with a diagnosis of miliaria crystallina. Miliaria, or “heat rash,” is characterized by the eruption of fine red papules and pustules specifically located on the back, typically after immobilization in the supine position. Miliaria is caused by superficial clogging of the eccrine sweat glands, which leads to the development of minute pustules that rupture easily and can be wiped off. The clogging may be partially due to overgrowth of Staphylococcus epidermidis. Treatment involves implementing measures to decrease sweating and topical measures such cool baths and the use of loose clothing.

Allergic contact dermatitis on the back would appear as an eczematous dermatitis with pruritus on the area exposed to the allergen. This patient does not have pruritus or clinical findings consistent with an allergic contact dermatitis.

Candida albicans infections occur in hot, moist occluded areas, such as the armpits, groin, and beneath the breasts. The rash is not usually diffuse and will typically have some desquamation at the edges. The papules cannot be wiped off as with miliaria crystallina.

Staphylococcal folliculitis will cause more deep-seated follicular pustules, and the area would be red, swollen, and painful. Although Staphylococcus aureus is responsible for a wide range of skin infections such as folliculitis, abscesses, furuncles, carbuncles, impetigo, cellulitis, ecthyma, staphylococcal scalded skin syndrome, and erysipelas, this patient's clinical presentation is not consistent with an infection with this organism.