A challenging EP tracing.

A 69-year-old male patient with prolonged PR interval presented with narrow QRS complex tachycardia for electrophysiological study. A discussion on different possibilities is made During tachycardia, diagnosis was made as atrioventricular nodal reentrant tachycardia, trial to ablate the slow pathway was not successful to cure the tachycardia along with a high degree atrioventricular block occurs made it more challenging for diagnosis, by studying the sequence of atrial, His and ventricular potentials diagnosis was supposed to be an accessory pathway connecting the atria with ventricle.