Congenital Melanocytic Nevus: This condition is characterized by hyperpigmented plaques and can be associated with hypertrichosis, especially in larger nevi. The presence of both hyperpigmentation and excessive hair growth within the nevus makes it a strong candidate for the single most likely diagnosis.

Becker's Nevus: A benign condition that presents as a hyperpigmented plaque, often with hypertrichosis. It typically appears in late childhood or adolescence and is more common in males.
Neurofibromatosis Type 1 (NF1): While NF1 is known for neurofibromas and café-au-lait spots, some patients may exhibit hyperpigmented plaques with hypertrichosis, particularly in the context of segmental NF1.
Epidermal Nevus Syndrome: This syndrome encompasses a range of conditions characterized by epidermal nevi, which can be hyperpigmented and hairy, along with other systemic manifestations.

Malignant Melanoma: Although less common, it's crucial to consider malignant melanoma, especially if there's a changing mole or an irregularly pigmented lesion with rapid growth or other suspicious features.
Phakomatosis Pigmentovascularis: A rare condition characterized by the coexistence of vascular and pigmentary birthmarks, which can include hyperpigmented plaques and potentially hypertrichosis in some cases.

Klippel-Trenaunay Syndrome: A rare congenital malformation that involves blood and lymph vessels, which can present with port-wine stains (but occasionally hyperpigmented plaques), varicose veins, and soft tissue hypertrophy, and less commonly, hypertrichosis.
Proteus Syndrome: A rare condition characterized by an overgrowth of skin, bones, and organs, which can include epidermal nevi and potentially hyperpigmented plaques with hypertrichosis among its myriad manifestations.