Amanda Richards is a 20-year-old junior in college. She ismajoring in biology and hopes someday to be a pediatrician.Beginning about a month ago, Amanda noticed that she was waking uponce, sometimes twice a night, by the need to go to the bathroom.More recently, she has noticed that she needs to go to the bathroomduring her school day much more frequently than before, sometimesas often as once every hour. At first Amanda thought that herincreased frequency of urination was due to the coffee she drank,but when she reduced her coffee consumption to one cup in themorning, she still needed to go to the bathroom just as often. Inaddition, Amanda was buying bottled water by the case, and shefound herself never without a beverage in her hand or nearby. Shealso noticed that her urine seemed pale and colorless. When Amandatold her mother of her problem, her mother became very concernedand arranged for Amanda to see the family physician. Her physicianfound no abnormalities on physical examination. However, a bloodchemistry profile revealed Amanda’s plasma sodium level to be 149mEq/L, plasma osmolarity was 308 mOsm/L, and her fasting plasmaglucose was 85 mg/dl. An analysis of Amanda’s urine showed a urineosmolarity of 200 mOsm/L. The urine sample was negative for thepresence of glucose. An extensive history revealed that no othermember of the family had ever displayed Amanda’s symptoms. Amandahad no history of traumatic head injury and an MRI of her brain wasnormal. Next, a two-hour water deprivation test was performed onAmanda. After two hours of not being able to drink water, theosmolarity of her plasma and urine were measured a second time.This time her urine osmolarity was unchanged; however, theosmolarity of her plasma increased to 315 from 308 mOsm/L. She wasthen injected with a drug called DDAVP, a synthetic analogue ofvasopressin. One hour after the injection, the osmolarity of herplasma decreased to 290 mOsm/L and the osmolarity of her urineincreased to 425 mOsm/L. Based upon the results above, Amanda’smedical history, and the results of the MRI, a diagnosis ofidiopathic pituitary diabetes insipidus (a form of central diabetesinsipidus) was made.
Why was a form of central or pituitary diabetes insipidusdiagnosed in Amanda’s case? (Assume she is not pregnant). Make sureto explain why the other types were ruled out.
