A 7-year-old girl has sparse, short, brittle scalp hair since infancy. Close inspection shows regular swellings along each shaft giving a beaded appearance, and the hair breaks at the constrictions. Light microscopy confirms periodic nodes. The underlying defect is:

• A Autoimmune destruction of the hair bulb matrix
• B Defective copper transport leading to abnormal keratin cross-linking
• C Abnormal disulfide bonding in the inner root sheath, X-linked recessive
• D Mutation of a hair cortex keratin gene, usually autosomal dominant ✓

Explanation

Monilethrix is a hereditary hair shaft anomaly with regularly beaded hair that fractures at the internodal constrictions. It is usually autosomal dominant and caused by mutations in type II hair keratin genes, classically KRT86 (hHb6) and KRT81 (hHb1). Menkes disease involves copper transport and causes kinky hair, not beads. The pattern of nodes plus constriction distinguishes it from trichorrhexis nodosa, which shows brush-like fractures.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.