The deciding feature is location plus colour: a deeply pigmented lesion on the heel/sole points to acral lentiginous melanoma, the melanoma variant that disproportionately affects palms, soles and nail beds in pigmented skin. Such lesions are clinically deceptive and frequently diagnosed late, so the workup must aim straight at a tissue diagnosis.
For any pigmented lesion suspected of melanoma, the gold standard is histopathological examination of an excisional biopsy. This not only confirms malignancy but yields the Breslow depth, the single most important prognostic determinant guiding margins and sentinel-node decisions.
The bedside stains offered are mismatched to the problem: a KOH preparation hunts fungal hyphae, a Gram stain hunts bacteria, and a Tzanck smear hunts acantholytic or viral giant cells - none can grade or confirm a melanocytic tumour.
\[\boxed{\text{Biopsy (histopathology) is the investigation of choice}}\]