An infant has downslanting palpebral fissures, malar and mandibular hypoplasia, malformed external ears, and conductive hearing loss with normal limbs and heart. The causative mutation lies in a gene required for ribosomal RNA biogenesis, whose failure triggers apoptosis of which cell population?

• A Cranial neural crest cells of pharyngeal arches 1 and 2 ✓
• B Somitic sclerotome cells of the occipital region
• C Surface ectoderm of the first branchial groove
• D Cardiac neural crest cells of the outflow tract

Explanation

Treacher Collins syndrome is caused by TCOF1 mutations that impair ribosomal DNA transcription, provoking p53 mediated apoptosis of cranial neural crest cells fated for the first and second arches, producing zygomatic and mandibular hypoplasia, ear anomalies, and conductive deafness. Cardiac neural crest is spared, so conotruncal defects and thymic aplasia do not occur, which distinguishes this from DiGeorge syndrome and kills the best distractor.

Reference: Larsen's Human Embryology, 5th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.