Final answer:
The priority nursing assessment for a patient about to begin digoxin therapy is to check the serum potassium level.
Step-by-step explanation:
The priority nursing assessment for a patient about to begin digoxin therapy would be to check the serum potassium level. Digoxin is a medication used to treat heart failure and abnormal heart rhythms, but it can cause side effects such as arrhythmias, which can be exacerbated by low potassium levels. Therefore, it is important to assess the patient's potassium level before starting digoxin therapy to ensure it is within the normal range.