QuestionSBA
QuestionChoice
This is incorrect, indeed the opposite is happening in this patient. ADH recruits aquaporin-2 channels in the distal tubule and collecting ducts of the kidney, thereby allowing water reabsorption. This patient's urine is too dilute, therefore the problem cannot be caused by increased secretion of ADH. Syndrome of inappropriate ADH secretion (SIADH) is a separate clinical entity characterised by increased ADH secretion causing excessive water retention and subsequently reduced serum osmolality.
10000383
c
Increased secretion of ADH
QuestionChoice
This patient is suffering from cranial diabetes insipidus. Several factors point towards this diagnosis: polyuria, increased serum osmolality, low urine osmolality, and urine osmolality responsive to desmopressin. Cranial DI occurs due to decreased anti-diuretic hormone (ADH) secretion from the pituitary gland. This results in impaired recruitment of aquaporin-2 channels in the distal tubule and collecting duct, resulting in an inability to concentrate urine.
10000381
a
Reduced anti-diuretic hormone secretion from the pituitary gland
QuestionChoice
Primary aldosteronism (AKA Conn's syndrome) is characterised by excess production of aldosterone, causing an excess exchange of potassium for sodium in the nephron. This results in hypernatraemia, hypertension, and hypokalaemia. However, this is unlikely to be the underlying cause as this patient demonstrates low urine osmolality which responds to desmopressin - a hallmark of cranial DI.
10000384
d
Excess aldosterone production
QuestionChoice
An important polyuria differential, though not the case here. T2DM is characterised by elevated plasma glucose. T2DM does not typically feature elevated serum osmolality combined with reduced urine osmolality and is therefore not the diagnosis.
10000385
e
Type 2 diabetes mellitus
QuestionChoice
Although this describes one potential mechanism of diabetes insipidus, impaired kidney response to ADH is the hallmark of nephrogenic DI. This patient is instead suffering from cranial DI. The two causes of diabetes insipidus can be differentiated by the desmopressin test. As this patient's kidneys have responded to desmopressin (an exogenous ADH analogue) as indicated by the subsequent significant increase in urine osmolality, renal response to ADH is intact. Therefore the pathology must lie elsewhere, confirming the diagnosis of cranial DI.
10000382
b
Impaired kidney response to anti-diuretic hormone
Concept
Chapter
2693
3754
Diabetes insipidus
Topic
133
Endocrinology
12623
A 51-year-old female presents to general practice with a 3-week history of polyuria. She reports no other urinary symptoms. Examination is normal apart from slightly reduced skin turgor. Further investigation reveals:

* Serum osmolality 312 mOsmol/kg (275-295 mOsmol/kg)
* Urine osmolality 91 mOsmol/kg (50-1200 mOsmol/kg)
* Urine osmolality 760 mOsmol/kg after administration of desmopressin

Which of the following best describes the pathophysiological mechanism behind this patient's condition?
a