Which statement best distinguishes sinus rhythm from atrial fibrillation?

• A. Atrial fibrillation has discernible P waves
• B. In sinus rhythm, P waves precede each QRS and are discernible; in atrial fibrillation, P waves are not discernible
• C. Atrial fibrillation has a regular R-R interval
• D. In sinus rhythm, QRS complexes are wide

Answer: (D)

Understanding the difference between sinus rhythm and atrial fibrillation hinges on atrial activity. In sinus rhythm you can see P waves that precede each QRS complex, and their timing is regular. In atrial fibrillation there are no distinct P waves at all; instead, the baseline is chaotic and the ventricular rhythm becomes irregularly irregular. This makes the presence of visible P waves a hallmark of sinus rhythm, while their absence points to atrial fibrillation.

The other statements don’t fit because: atrial fibrillation does not have discernible P waves, and its R-R intervals are not regular but irregular; wide QRS complexes are not a defining feature of sinus rhythm (they’re usually due to ventricular conduction problems).