A 34-year-old man has recurrent syncope occurring exclusively at rest and during sleep. Baseline ECG shows a QTc of 500 ms with no unusual T wave morphology. Genetic testing reveals a gain-of-function mutation affecting the cardiac sodium channel. Which drug is specifically useful in addition to a beta-blocker in this genotype?

• A Verapamil
• B Flecainide
• C Quinidine
• D Mexiletine ✓

Explanation

This is LQT3, caused by gain-of-function mutations in SCN5D producing persistent late sodium current and prolonged repolarization. Events characteristically occur at rest or during sleep, unlike the exercise-triggered events of LQT1. Mexiletine, a sodium channel blocker, shortens the action potential duration and is the genotype-specific adjunct to beta-blockade. Flecainide and quinidine are not standard LQT3 therapy.

Reference: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 12th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.