Bowen Disease SCC In Situ

A 72-year-old fair-skinned woman presents with a slowly enlarging lesion on her lower leg that has been present for 2 years. On examination, the structure marked **A** — a well-demarcated erythematous plaque with adherent scale — is noted. Histopathology reveals full-thickness keratinocyte atypia with loss of normal maturation, abundant mitoses at all levels, and dyskeratotic cells, but the basement membrane remains intact. Which of the following best describes the pathological basis of this lesion?