During which condition is a wide QRS complex most likely to occur?

A wide QRS complex is most likely to occur in the presence of a bundle branch block. In this condition, there is a delay in the conduction of electrical impulses through one of the bundle branches in the heart, which leads to a prolonged duration of ventricular depolarization. As a result, the QRS complex on the electrocardiogram (ECG) becomes wider than normal.

In comparison, normal sinus rhythm and sinus bradycardia typically exhibit a narrow QRS complex, as the electrical impulses are conducted efficiently through the normal conduction pathways. First-degree AV block, while it may prolong the PR interval, does not typically affect the width of the QRS complex, which remains narrow unless there is an underlying conduction delay present in either of the bundle branches.

Thus, the presence of a bundle branch block directly correlates with the occurrence of a wide QRS complex due to the disrupted and delayed conduction of impulses through the heart's ventricular system.