It will never be SINUS rhythm when?

• A. P waves are not 1 per QRS, upright, and similar in shape and size
• B. P waves are 1 per QRS upright, and similar in shape and size
• C. The rhythm is regular
• D. QRS complex is narrow

Answer: (A)

Sinus rhythm hinges on atrial activation starting in the sinoatrial node, which produces one P wave before every QRS complex, with the P waves upright and similar in shape and size, and the rhythm usually regular. If the P waves are not one per QRS, or they don’t precede each QRS in a consistent, uniform way (or are absent/inverted), the rhythm cannot be sinus because the atrial activation is not linked cleanly to each ventricular contraction.

In this item, the statement that P waves are not 1 per QRS, upright, and similar in shape and size violates that essential 1:1, orderly P-to-QRS relationship, so it cannot be sinus rhythm. The other statements describe features compatible with sinus rhythm: a single, upright, uniform P wave before each QRS, a regular rhythm, and a narrow QRS.