A 19-year-old presents with hundreds of colonic adenomas and a desmoid tumour of the anterior abdominal wall. Genetic testing shows an APC mutation. Which site of APC mutation best explains the association with severe polyposis and desmoid disease?

• A Codon 1061
• B Mutation between codons 1445 and 1578
• C Codon 1309 ✓
• D Promoter region 1B

Explanation

Mutations at codon 1309 produce the severest phenotype: early-onset, high-density polyposis with colorectal cancer by the late teens, and are linked to intra-abdominal desmoid tumours. Desmoid risk is classically associated with mutations distal to codon 1400, but 1309 remains the textbook answer for severe polyposis. Codon 1061 gives ordinary classic FAP, and promoter 1B involvement causes attenuated FAP with fewer polyps and later onset.

Reference: Sabiston Textbook of Surgery, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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