The ECG shows Mobitz type I (Wenckebach) second degree AV block with variable conduction. You can see both 2:1 and 4:3 conduction patterns. Look closely for the classic progressive PR lengthening before a dropped beat.

I've marked the P waves on the ECG to help you track the
rhythm more easily.

Clinical clue: In an older patient without prior medical history, this is most likely due to age-related degeneration of the AV node. But remember, whenever you see a second-degree AV block, it's important to also rule out other causes like ischemia, electrolyte disturbances, medications, or increased vagal tone.