Folliculitis: This is the most straightforward diagnosis given the description. Folliculitis is an inflammatory condition affecting the hair follicles, often caused by bacterial (most commonly Staphylococcus aureus) or fungal infections. The posterior thigh is a common location due to friction and moisture.

Keratosis Pilaris: A condition characterized by small, rough, sandpapery bumps or patches on the skin, often on the thighs, due to the accumulation of keratin. It can resemble folliculitis but is usually not inflammatory.
Pityriasis Rosea: Although it typically starts with a herald patch, the subsequent rash can appear on the thighs and may resemble folliculitis, especially if the patient has not noticed the initial patch.
Contact Dermatitis: An allergic or irritant reaction to substances coming into contact with the skin, which can cause a rash that might be mistaken for folliculitis, especially if it involves the hair follicles.
Eczema (Atopic Dermatitis): Can cause a rash that might resemble folliculitis, particularly if the eczema is localized to the posterior thighs and involves the follicles.

Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can start as a small, seemingly insignificant lesion but can rapidly spread and become serious. It requires prompt antibiotic treatment.
Necrotizing Fasciitis: A severe infection of the tissue beneath the skin, including fat and the tissue covering the muscles (fascia). It's much rarer but can be life-threatening if not treated immediately.
Methicillin-Resistant Staphylococcus aureus (MRSA) Infection: MRSA can cause folliculitis but also more severe infections like cellulitis or abscesses. It's resistant to many antibiotics, making it a critical diagnosis not to miss.

Eosinophilic Folliculitis: A condition more commonly seen in individuals with HIV/AIDS, characterized by an itchy, follicular papular rash.
Gram-Negative Folliculitis: Typically occurs in individuals with long-standing acne treated with antibiotics, where the normal flora is disrupted, allowing gram-negative bacteria to overgrow and cause folliculitis.
Malassezia Folliculitis: Caused by the yeast Malassezia, which is part of the normal skin flora. It can cause folliculitis, especially in oily areas of the skin and in individuals who use oily skin products or have immunosuppression.