A nurse is caring for a client who is taking lithium. Which of the following findings should the nurse monitor for as an adverse effect of the medication?

The correct answer is D: Increased urinary output. Lithium is known to cause nephrogenic diabetes insipidus, leading to increased urinary output. The rationale is that lithium impairs the kidney's ability to concentrate urine, resulting in excessive urine production. Monitoring urinary output is crucial to detect this adverse effect early. Choices A, B, and C are incorrect because positive Chvostek's sign is associated with hypocalcemia, increased potassium level is not a typical adverse effect of lithium, and constipation is not directly related to lithium use.