Answer: D

The correct answer is fasciculoventricular (FV) pathway and electrophysiological study should be the next step of management. Figure 1 demonstrates a 12-lead ECG showing sinus rhythm with a slightly short PR interval of 112 ms and evidence of pre-excitation, especially in precordial leads. The right lateral pathway would have a negative delta wave in lead V1 and left lateral pathway would have it in lead I and aVL. Figure 2 demonstrates a junctional rhythm on the first three beats and sinus rhythm on the last three beats of tracing. There is pre-excitation present as seen by slurring in upstroke of leads V2, V3, V4, and limb leads. Of note, the degree of pre-excitation is same in the sinus and junctional beats. In a typical atrioventricular accessory pathway, junctional beats will not show any pre-excitation since the depolarisation starts below the atrium and does not engage the accessory pathway. Hence, the finding of the similar degree of pre-excitation for junctional and sinus beat is diagnostic for FV pathway. Her electrophysiological study confirmed this diagnosis with a fixed HV interval. In addition, she had easily inducible atrioventricular nodal re-entry tachycardia which most likely caused her palpitation. Successful ablation of the slow pathway was performed. A single chamber defibrillator was implanted for prevention of sudden cardiac death from HCM. FV pathway had never been demonstrated to be the key component of re-entrant tachycardia due to its short distance and considered to be benign.1 Its pre-excitation pattern could mimic anteroseptal or mid-septal pathway and lead to the unnecessary risk of complete heart block with an attempted catheter ablation. Danon disease is an X linked dominant lysosomal storage disease derived from the genetic defects in LAMP gene mutation with multiorgan involvement.2 3 Pre-excitation is very common and risk for sudden cardiac death is high3 4 in patients with Danon disease.