The approach to clinical conundrums by an expert clinician is revealed through presentation of an actual patient's case in an approach typical of morning report. Similar to patient care, sequential pieces of information are provided to the clinician who is unfamiliar with the case. The focus is on the thought processes of both the clinical team caring for the patient and the discussant. A 71-year-old man presented to a hospital with a one week history of fatigue, polyuria, and polydipsia. He also reported pain in his back, hips, and ribs, in addition to frequent falls, intermittent confusion, constipation, and a weight loss of 10 pounds over the last 2 weeks. He denied cough, shortness of breath, chest pain, fever, night sweats, headache, and focal weakness.Polyuria, which is often associated with polydipsia, can be arbitrarily defined as a urine output exceeding 3 L per day. After excluding osmotic diuresis due to uncontrolled diabetes mellitus, the 3 major causes of polyuria are primary polydipsia, central diabetes insipidus, and nephrogenic diabetes insipidus. Approximately 30% to 50% of cases of central diabetes insipidus are idiopathic; however, primary or secondary brain tumors or infiltrative diseases involving the hypothalamic-pituitary region need to be considered in this 71-year-old man. The most common causes of nephrogenic diabetes insipidus in adults are chronic lithium ingestion, hypokalemia, and hypercalcemia. The patient describes symptoms that can result from severe hypercalcemia, including fatigue, confusion, constipation, polyuria, and polydipsia.The patient's past medical history included long-standing, insulinrequiring type 2 diabetes with associated complications including coronary artery disease, transient ischemic attacks, proliferative retinopathy, peripheral diabetic neuropathy, and nephropathy. Seven years prior to presentation, he received a cadaveric renal transplant that was complicated by BK virus (polyomavirus) nephropathy and secondary hyperparathyroidism. Three years after his transplant surgery, he developed squamous cell carcinoma of the skin, which was treated with local surgical resection. Two years after that, he developed stage I laryngeal cancer of the glottis and received laser surgery, and since then he had been considered disease-free. He also had a history of hypertension, hypercholesterolemia, osteoporosis, and depression. His medications included aspirin, amlodipine, metoprolol succinate, valsartan, furosemide, simvastatin, insulin, prednisone, sirolimus, and sulfamethoxazole/trimethoprim. He was a married psychiatrist. He denied tobacco use and reported occasional alcohol use.The prolonged immunosuppressive therapy that is required following organ transplantation carries a markedly increased risk of the subsequent development of malignant tumors, including cancers of the lips and skin, lymphoproliferative disorders, and bronchogenic carcinoma. Primary brain lymphoma resulting in central diabetes insipidus would be unlikely in the absence of headache or focal weakne...