An infant has a white forelock, a sharply demarcated depigmented patch on the forehead and anterior trunk containing islands of normally pigmented and hyperpigmented skin within it, and bilateral iris heterochromia. The patches have remained unchanged in size since birth. The underlying defect is in:

• A TYR causing absent tyrosinase activity
• B PAX3 causing neural crest migration failure
• C KIT proto-oncogene affecting melanoblast migration ✓
• D MITF causing melanocyte differentiation arrest

Explanation

Piebaldism is autosomal dominant, caused by KIT mutations impairing melanoblast survival and migration, giving stable ventral depigmented patches with hyperpigmented islands and a white forelock from birth. Distinguishing it from vitiligo is the stability since birth and the presence of pigment islands. TYR mutations cause oculocutaneous albinism type 1 with generalised hypopigmentation rather than patterned patches. MITF and PAX3 defects cause Waardenburg syndrome, which adds deafness and dystopia canthorum.

Reference: Rook's Textbook of Dermatology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.