<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 30-year-old man comes to the clinic with complaints of increased frequency of urination, especially at night, for about a month. He has to wake up at least 5-6 times every night to urinate and this is negatively affecting his sleep. He also complains of increased thirst and generalized weakness. Past medical history is significant for bipolar disorder. He is on lithium which he takes regularly. Blood pressure is 15090 mm Hg, pulse rate is 80min, respiratory rate is 16min, and temperature is 36C 96.8F. Physical examination is normal. Laboratory studies show:Na: 146 mEqLK: 3.8 mEqLSerum calcium: 9.5 mgdLCreatinine: 0.9 mgdLUrine osmolality: 195 mOsmkgSerum osmolality: 305 mOsmkgSerum fasting glucose: 90 mgdLWhich of the following is the best initial test for the diagnosis of his condition?? A: MRI scan of brain, B: CT thorax, C: Chest X-ray, D: Water deprivation test, E: Serum ADH level,  <response>: Based on the symptoms of increased urination polyuria especially at night, increased thirst polydipsia, and the lab findings of low urine osmolality compared to serum osmolality, the most likely diagnosis is diabetes insipidus DI. This is characterized by the inability to concentrate urine leading to excessive urination and thirst. The first step in evaluating DI is to do a water deprivation test, where fluid intake is restricted and urine osmolality is measured over time. Failure of the urine to concentrate indicates DI. This helps distinguish nephrogenic DI due to renal defects in responding to ADH from centralneurogenic DI due to lack of ADH secretion. MRI brain or checking serum ADH levels can help diagnose central DI by looking for structural abnormalities in the pituitaryhypothalamus or confirming inappropriately low ADH levels. But these are not the best initial tests. CT thorax and CXR are unrelated to the presentation.D: Water deprivation test 