A 13-year-old boy with patchy depigmented skin on the right flank and upper thigh in segmental distribution. The
depigmentation started 1 year back but has been static for last 4 months. Mother reports use of topical steroids which was
ineffective.
Segmental vitiligo.

Vitiligo is an acquired autoimmune condition targeting melanocytes and present with localized or widespread white depigmented patches. Thyroid autoimmune dysfunction is associated.
Piebaldism is an autosomal dominant condition characterized by white forelock and circumscribed depigmented patches affecting the body. It is caused by a defect in proliferation and migration of melanocytes during embryo- genesis. Unlike vitiligo, it is congenital and non-progressive.
In the question the depigmentation started at age of 12 years and patient presented at 13 years.
Nevoid hypomelanosis is characterized by hypopigmented patches or streaks which follow the lines of Blaschko. They are present at birth and may develop in the first 2 years of life.
Segmental vitiligo
Piebaldism
Hypomelanosis of Ito
Hypopigmented streaks