Long QT Syndrome Type 2 - KCNH2 Auditory-Triggered

A 22-year-old woman on postpartum day 10 presents with syncope triggered by her infant's cry at 3 AM. She reports a prior episode when a smoke alarm sounded and another during a shower when the doorbell rang. Her brother died suddenly at age 32 during sleep after his alarm clock went off. Physical examination is unremarkable. The 12-lead ECG shows the pattern marked **A** in the diagram: prolonged QTc of 561 ms (normal female <450 ms) with distinctive low-amplitude bifid/notched T-waves best seen in leads II, V4, and V5. Serum potassium is 3.6 mEq/L and magnesium is 1.7 mg/dL. Which genetic mutation and ion channel defect best explains this clinical presentation and ECG pattern?