Final answer:
In a client with diabetes insipidus, the onset of confusion and lethargy is most critical to report as it indicates a potential critical change in the patient's status, possibly related to severe dehydration or electrolyte imbalances.
Step-by-step explanation:
The most important information for the nurse to report to the health care provider when caring for a client with diabetes insipidus (DI) is b. Confusion and lethargy. Lethargy and confusion can be signs of significant electrolyte imbalances or severe dehydration, which require immediate medical attention. While the history of recent head injury, low specific gravity of the urine, and high urine output are also noteworthy, they are more consistent with what's expected in DI and may not signal an acute change in the patient's status.
In diabetes insipidus, a low urine specific gravity such as 1.003 indicates dilute urine, which is characteristic of the disease. Urine output of 400ml/hour indicates polyuria, another common symptom of DI. However, the sudden onset of confusion and lethargy represents an acute risk to the patient's well-being and may suggest a critical need for intervention to prevent further complications.