A 78-year-old man presents with dyspnea on exertion and fatigue, showing an irregularly irregular heart rhythm on ECG. What is the likely diagnosis?

The presence of dyspnea on exertion, fatigue, and an irregularly irregular heart rhythm on ECG strongly suggests atrial fibrillation as the likely diagnosis. Atrial fibrillation is characterized by chaotic electrical activity in the atria, leading to an irregular and often rapid heart rate. Patients frequently experience symptoms such as fatigue and shortness of breath, especially during exertion, due to the inefficient pumping of blood and the resultant reduced cardiac output.

In this case, the patient is an older adult, which is a population at higher risk for developing atrial fibrillation. The irregularly irregular rhythm observed on the ECG confirms the diagnosis, as atrial fibrillation typically presents with a lack of a discernible P wave and an irregular ventricular response.

While myocardial infarction and pulmonary embolism could potentially lead to similar symptoms of dyspnea and fatigue, they usually present with additional specific signs or symptoms. Congestive heart failure could also be a consideration given the dyspnea and fatigue, but the distinct irregular heart rhythm on ECG specifically points to atrial fibrillation as the underlying cause in this scenario. Thus, the combination of irregular heart rhythm and clinical presentation of exertional dyspnea and fatigue supports the diagnosis of atrial fibrillation.