A 56-year-old man comes to the emergency department due to chest palpitations. The patient feels that his heartbeat is fast and irregular. He has had no chest pain, shortness of breath, or dizziness. The patient normally drinks only 2-3 times a year, but he hosted a party last night for his wife's birthday and consumed a large amount of alcohol. He is otherwise healthy and takes no medications. Pulse check confirms the presence of an irregularly irregular rhythm with a rate of 138/min. This patient's ECG strip is most likely to show which of the following?