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Widened QRS complexes on an ECG are primarily associated with conditions that impair the normal conduction pathways within the heart, particularly in the ventricles. Bundle branch block is one such condition where there's a delay in the conduction through one of the bundle branches — either the right or left. This delayed conduction causes the ventricles to contract asynchronously, resulting in a longer duration of the QRS complex.

In contrast, atrial fibrillation typically presents with unrecognizable QRS complexes and irregular rhythms but does not predominantly cause QRS widening. Myocardial infarction may lead to various changes on an ECG, including ST segment changes and possibly wide QRS complexes, but this is not the typical primary finding and would usually be part of a broader ECG interpretation. Aortic stenosis can cause changes due to increased left ventricular hypertrophy and impaired systolic function, but again, this does not directly relate to widened QRS complexes.

Therefore, when considering the conditions that specifically lead to widened QRS complexes, bundle branch block stands out as the most likely cause due to its direct effect on the electrical conduction through the ventricles.