Final answer:
The nurse should withhold digoxin and notify the provider if the client is experiencing visual disturbances, as it could indicate toxic effects.
Step-by-step explanation:
The nurse should withhold the medication and notify the provider if the client is experiencing visual disturbances. Visual disturbances are a potential side effect of digoxin and can indicate toxicity. The other findings mentioned - serum potassium level of 3.8 mEq/L, heart rate of 90 beats per minute, and blood pressure of 130/80 mm Hg - are within normal ranges and would not require withholding the medication.