Dermatosis papulosa nigra or frictional melanosis: This condition is characterized by darkened skin due to friction or pressure, commonly seen in areas like the medial ankles. It's benign and more common in darker-skinned individuals.

Erythema dyschromicum perstans: A condition that causes progressive, asymptomatic hyperpigmentation, often starting in childhood. It can appear as blue-gray patches and can be mistaken for dirt.
Fixed drug eruption: Although less common in children, this condition can cause well-defined patches of hyperpigmentation following exposure to certain medications.
Tinea nigra: A superficial fungal infection that causes brown or black spots on the skin, typically on the palms or soles, but can appear elsewhere, including the ankles.

Addison's disease: A rare endocrine disorder that can cause hyperpigmentation, among other symptoms like fatigue, weight loss, and low blood pressure. Early diagnosis is crucial for treatment.
Hemochromatosis: A genetic disorder leading to iron overload, which can cause skin darkening (bronze diabetes). It's rare in children but critical to diagnose early to prevent organ damage.
Malignancy (e.g., melanoma): Although extremely rare in children, any new, unusual skin lesion should be evaluated for malignancy.

Arteriovenous malformation: Abnormal connections between arteries and veins can lead to localized skin changes, including hyperpigmentation, due to altered blood flow.
Neurofibromatosis type 1 (NF1): A genetic disorder that can cause skin changes, including café-au-lait spots, which are hyperpigmented lesions.
Congenital melanocytic nevi: Benign melanocytic proliferations present at birth, which can appear as hyperpigmented areas and have varying degrees of risk for melanoma development.