Familial Cylindromatosis (CYLD)

A 35-year-old woman presents with multiple firm, dome-shaped pink-to-blue nodules confluent across the scalp, forming a characteristic "turban-like" appearance. Similar lesions have appeared on her face and trunk over the past 5 years. Her mother and maternal grandmother had identical presentations. The pedigree shown in the diagram (marked **A**) demonstrates autosomal dominant inheritance with female-predominant expression and vertical transmission across generations. Which of the following best explains the molecular basis of this condition?