A 24 year old male attends his GP practice due to a persistent issue with excessive urination and excessive thirst. He feels otherwise well and denies fever, weight loss and fatigue. There is no family history of renal disease.

On examination, there is no central or peripheral oedema. Observations are within normal limits. Blood test results, including serum glucose and renal function tests, are unremarkable aside from a mild hypernatraemia. However, he showed no increase in urine osmolality even after dehydration and desmopressin (DDAVP) administration in a fluid deprivation test.

Which of the following is the most likely diagnosis?

Options:
explanation: In patients with cranial diabetes insipidus, a fluid deprivation test typically results in an increase in urine osmolality after the administration of DDAVP. This is because cranial diabetes inspidus is characterised by a deficiency of ADH production in the brain. Administering DDAVP (synthetic ADH) during the test usually corrects the defect, and urine osmolality increases. In contrast, in nephrogenic diabetes inspidus, urine osmolality remains low even after DDAVP administration because the kidney tubules do not respond to ADH. Therefore, nephrogenic diabetes insipidus is the correct answer.
name: Cranial diabetes inspidus
label: b
answer: no
explanation: Patients with psychogenic polydipsia can produce concentrated urine when they are deprived of fluids. In this vignette, the patient's urine remained dilute even after dehydration and the administration of DDAVP, which suggests a problem with the kidney's response to ADH rather than a psychological issue. The correct answer is nephrogenic diabetes insipidus, a rare condition characterised by a defect in the kidney's ability to respond to ADH, leading to the inability to concentrate urine and resulting in excessive thirst and polyuria.
name: Psychogenic polydipsia
label: e
answer: no
explanation: This is the correct answer. Based on the findings of the fluid deprivation test, the patient's inability to concentrate his urine even after DDAVP administration suggests a diagnosis of nephrogenic diabetes insipidus. This is a rare condition characterised by a defect in the kidney's ability to respond to ADH, leading to the inability to concentrate urine. Patients commonly present with excessive thirst and polyuria, as in this case.
name: Nephrogenic diabetes inspidus
label: a
answer: yes
explanation: Although renal function tests are an important investigation, they were normal in this patient. This is also indicated by the absence of oedema, the young age of the patient and the absence of a family history of renal diseases. Based on the findings of the fluid deprivation test, this patient's inability to concentrate his urine even after DDAVP administration suggests a diagnosis of nephrogenic diabetes insipidus. This is a rare condition characterised by a defect in the kidney's ability to respond to ADH, leading to the inability to concentrate urine and resulting in excessive thirst and polyuria.
name: Chronic kidney disease
label: d
answer: no
explanation: Osmotic diuresis occurs when there are high levels of solutes, such as glucose, calcium and urea in the urine that cause an osmotic gradient, drawing water into the urine. This results in increased urine output. In this vignette, the normal blood tests, particularly normal glucose level, rule out osmotic diuresis. Based on the findings of the fluid deprivation test, this patient's inability to concentrate his urine even after DDAVP administration suggests a diagnosis of nephrogenic diabetes insipidus. This is a rare condition characterised by a defect in the kidney's ability to respond to ADH, leading to the inability to concentrate urine and resulting in excessive thirst and polyuria.
name: Osmotic diuresis
label: c
answer: no

Correct answer: a