Tinea pedis (Athlete's foot): A common fungal infection of the foot, often presenting with brown spots or patches, especially on the ankles and feet. The lack of other symptoms such as itching or burning could be due to a mild or early infection.

Erythema dyschromicum perstans: A condition characterized by the sudden onset of brown to gray macules, often on the face, neck, and upper limbs, but can also appear on the ankles and feet. It's more common in darker-skinned individuals.
Pigmented purpuric dermatosis (Schamberg's disease): A chronic condition presenting with petechiae and brown pigmentation, typically on the lower limbs. It's often asymptomatic aside from the rash.
Stasis dermatitis: Caused by poor circulation, leading to fluid buildup and skin discoloration, often seen in the lower legs and ankles. The brown spots could be a result of hemosiderin deposition.

Malignant melanoma: Although rare, a new or changing brown spot, especially with irregular borders, color variation, or diameter larger than 6mm, warrants concern for melanoma. Early detection is crucial for treatment.
Kaposi's sarcoma: A type of cancer that can cause brown or purple lesions on the skin, often associated with immune suppression (e.g., HIV/AIDS). It's essential to consider in individuals with risk factors.

Argyria: A condition caused by silver exposure, leading to blue-gray pigmentation of the skin and mucous membranes. While it's rare and the color is typically more blue than brown, it could be considered in individuals with a history of silver ingestion or exposure.
Chromoblastomycosis: A fungal infection of the skin and subcutaneous tissue, more common in tropical and subtropical regions. It can cause brown to black lesions, typically after a traumatic inoculation of the fungus.