This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fluid and electrolytes, Subject name: Medicine
After administering 'desmopressin', urine osmolality should increase by how much in central diabetes insipidus? 
Select and explain. 
A. At least 10%, B. At least 25%, C. At least 35%, D. At least 50%
D: At least 50%
Answer: D: At least 50% 
Explanation: CDI & NDI can be distinguished by administering AVP analogue desmopressin (10 mg intranasally) after careful water restriction. Urine osmolality should increase by at least 50% in CDI & will not change in NDI. Patients with NDI will fail to respond to DDAVP, with a urine osmolality that increases by <50% or <150 mOsm/kg from baseline, in combination with a normal or high circulating AVP level; Patients with central DI will respond to DDAVP, with a reduced circulating AVP. Patients may exhibit a paial response to DDAVP, with a >50% rise in urine osmolality that nonetheless fails to reach 800 mOsm/kg; the level of circulating AVP will help differentiate the underlying cause, i.e., NDI versus central DI. Ref: Harrison 19e pg: 302