A 35-year-old woman from Kerala presents with a 2 cm dark nodule on her sole that has been enlarging over 3 months. Dermoscopy reveals parallel ridge pattern with brown-black pigmentation. Histopathology shows melanoma with Breslow thickness of 3.2 mm and 8 mitoses per mm². Which feature BEST distinguishes acral lentiginous melanoma from superficial spreading melanoma in this patient?