Final answer:
The nurse should prioritize assessing the client with a myocardial infarction (MI) exhibiting an S3 heart sound as it can indicate imminent heart failure, which is a critical and potentially life-threatening condition.
Step-by-step explanation:
The priority for the nurse in the telemetry unit would be to assess the client who has the most critical and potentially life-threatening condition first. An S3 heart sound in a client with a myocardial infarction (MI) can indicate heart failure, which requires immediate attention. While a client with chronic heart failure (CHF) and severe sacral pitting edema or a client with chronic renal failure and elevated creatinine levels also present significant concerns, these conditions typically do not progress as rapidly as complications of an MI. A patient with pneumonia and a pulse ox reading of 94% is generally considered stable, as normal pulse oximetry readings range from 95% to 100%; however, this could indicate the onset of respiratory problems and should not be ignored, but it is not as urgent as the potential heart failure in the MI patient.