Final answer:
The nurse's best first action is to hold the next dose of the prescribed antidiarrheal drug.
Step-by-step explanation:
The nurse's best first action to prevent harm in a client with severe diarrhea reporting tingling lips and foot cramps would be A. Hold the next dose of the prescribed antidiarrheal drug.
Tingling lips and foot cramps can be suggestive of low calcium levels, known as hypocalcemia. Severe diarrhea can lead to electrolyte imbalances, including hypocalcemia. By holding the antidiarrheal drug, the nurse can prevent further loss of electrolytes and potentially worsening the client's condition.