Step 1: Well-demarcated chalky-white patches caused by loss of melanin define leukoderma, literally white (leuko) skin (derma).
Step 2: The depigmented areas can enlarge and coalesce, and in advanced disease they may involve the face, scalp and genital skin extensively.
Step 3: Leukoderma is the general term for melanin loss, with vitiligo being one autoimmune subtype, so the over-arching diagnosis matching the picture is leukoderma.
Step 4: Piebaldism is present from birth with a white forelock, while discoid lupus scars and scales rather than simply depigmenting, so both are set aside.
\[\boxed{\text{Leukoderma}}\]