Final answer:
The nurse's priority for a 4-month-old infant with heart failure on digoxin should be the capillary refill time of 4 seconds, indicating potential circulatory compromise.
Step-by-step explanation:
The nurse's priority in admitting a 4-month-old infant who has heart failure and is taking digoxin should be to address the capillary refill time of 4 seconds. While a heart rate of 110 beats per minute and a respiratory rate of 32 breaths per minute are within normal ranges for an infant, a capillary refill time of more than 2 seconds is considered prolonged and can indicate poor perfusion, which might be a sign of decompensating heart failure. Furthermore, a serum potassium level of 3.2 mEq/L, although slightly low (as hypokalemia can in rare cases potentiate digoxin toxicity), is not as immediately concerning as the capillary refill indicating potential circulatory compromise. Therefore, the capillary refill time is the finding that requires the most urgent attention in this scenario.