Final answer:
The electrocardiogram (ECG or EKG) is the most accurate method to assess a client's heart rate and rhythm for cardiac dysrhythmia, as it provides a detailed electrical representation of heart function.
Step-by-step explanation:
The most accurate method for assessing a client's heart rate and rhythm, especially when there is a known cardiac dysrhythmia, is by using an electrocardiogram (ECG or EKG). An ECG measures the electrical impulses of the heart, providing a detailed representation of the heart's rhythm. While auscultation using a stethoscope is a simpler method, it may not provide the detailed data required for analyzing complex dysrhythmias. The ECG trace can also be used to calculate the heart rate by assuming a constant time between beats, which is particularly useful for determining the heart rhythm and identifying any irregularities.