Final answer:
A nurse should monitor the client receiving valproic acid and taking aspirin for bruising of the skin, due to the interference of aspirin with platelet function.
Step-by-step explanation:
The client receiving valproic acid (Depakene) and also taking 81 mg of aspirin daily should be monitored for bruising of the skin. As aspirin is known to interfere with the action of platelets, leading to an increased risk of bruising due to its effect on prostaglandin biosynthesis and inhibition of thromboxane A2, a potent agent for platelet aggregation. Therefore, in the context of the given options, a nurse should be most vigilant for signs of bruising, which might indicate an increased bleeding risk for the client.