Red patches on the crotch can be caused by psoriasis, lichen planus, neurodermatitis, ringworm and other diseases. 1. psoriasis: psoriasis, also known as psoriasis, can occur in the crotch, red papules may appear in the early stages of the disease, and as the disease progresses, it can be fused into a clear border of red plaques. 2. Lichen planus: the cause of lichen planus is still unclear, it can occur on the bare feet, wrists, inner thighs, resulting in localized purplish-red lesions, flat papules, plaques. 3. Neurodermatitis: the cause of neurodermatitis is not completely clear, can lead to localized pimples or moss-like changes. Frequent scratching of the local area leads to the appearance of scratches and scabs on the skin. 4. Tinea corporis: a fungal infectious disease that can lead to ring-shaped red spots on the crotch, covered with a thin layer of scales, sometimes accompanied by obvious itching. The appearance of red patches on the crotch is considered to be related to a variety of diseases, and it is recommended to go to the dermatology department of a regular hospital for treatment under the guidance of a doctor.