A 50-year-old man is evaluated for a firm, darkly pigmented papule on his back that has been growing steadily over a period of several months. His medical history is unremarkable, and he takes no medications.
On physical examination, vital signs are normal. The skin lesion is a 0.5-cm darkly pigmented, raised plaque with irregular borders and inconsistent coloration. The remainder of his physical examination, including the rest of his skin examination, is unremarkable.
A skin biopsy is performed and shows malignant melanoma on pathologic examination.
Which of the following is the primary feature used to determine prognosis?