Final answer:
Decreased potassium levels (Hypokalemia) are likely to cause the prolonged PR interval and widened QRS complex observed in the cardiac monitoring.
Step-by-step explanation:
A nurse caring for a client requiring continuous cardiac monitoring has identified a prolonged PR interval and widened QRS complex on the ECG. Among the listed lab values, C) Decreased potassium levels support this finding. Hypokalemia, which is the term for low potassium levels, can lead to arrhythmias as maintaining electrolyte balance is critical for the normal wave of depolarization in cardiac function. None of the other listed lab values, such as elevated troponin (which often indicates myocardial infarction), elevated creatinine (associated with kidney function), or increased magnesium levels, are typically associated with the ECG changes observed here, such as the prolonged PR interval and widened QRS complex.