Final answer:
Polydipsia, or excessive thirst, would indicate that the client is at risk for diabetes insipidus (DI) in the context of a closed head injury. This symptom is the body's way of compensating for the excessive loss of water through urine.
Step-by-step explanation:
Diabetes insipidus (DI) is a condition characterized by excessive thirst (polydipsia) and excretion of large amounts of diluted urine. In the case of a client with a closed head injury, certain findings may indicate a risk for DI. Polydipsia, which refers to excessive thirst, would be an indicator that the client is at risk for DI. Increased thirst is a common symptom of DI as the body tries to compensate for the loss of water through excessive urine output. Therefore, the presence of polydipsia would be a significant finding for the nurse to recognize and assess.