The causes of a block of mid-brown spots on the skin of the lower leg include flat warts, melanoma, and seborrheic keratosis. 1. flat warts: is caused by the human papilloma virus (HPV) infection of the skin mucous membrane of the benign redundancy, typical skin lesions for the rice grain to soybean size flat elevated pimples, round or oval, the surface is smooth, hard, normal skin color or light brown. 3. Melanoma: If the stimulation of various factors on the calf triggers a large number of melanocyte proliferation, brown spots will also appear. 4. Seborrheic keratosis: Seborrheic keratosis is the most common benign epidermal hyperplastic tumor in the elderly, the lesion is one or several yellowish or light brown, slightly higher than the skin surface of the flat papules or plaques. Calf skin has a block of medium brown spots of more causes, it is recommended to go to the hospital in time to check, standardized treatment.