Diabetes insipidus: The Daily PANCE Blueprint

The answer is A. Central Diabetes Insipidus

Diabetes Insipidus (DI) is a deficiency of or resistance to Antidiuretic Hormone (ADH/Vasopressin). In Central DI, the posterior pituitary fails to secrete ADH, usually due to head trauma, tumor, or idiopathic causes. This leads to the inability of the kidneys to concentrate urine, causing massive polyuria and dilute urine.

• Diagnosis: The Vasopressin (Desmopressin) Challenge is the gold standard for differentiation.
  • If urine Osmolarity increases significantly (>50%) after Desmopressin, it confirms the kidneys can respond to ADH, meaning the problem was a lack of production (Central DI).
• Treatment: The treatment of choice for Central DI is Desmopressin (DDAVP), a synthetic analog of ADH.
• Why the others are wrong:
  • Nephrogenic DI: The kidneys are resistant to ADH. There would be no response (or <10% increase in urine Osm) after giving Desmopressin. Often caused by Lithium use.
  • Primary Polydipsia: Caused by excessive water intake. During a water deprivation test, these patients will concentrate their urine naturally because their ADH production and response are normal.
  • Type 2 Diabetes Mellitus: While it causes polyuria, the urine would have a high specific gravity due to glucose (glycosuria), not low osmolarity.
  • SIADH: This is the “opposite” of DI; it involves too much ADH, leading to concentrated urine and hyponatremia, not hypernatremia.

Board Pearl: Central DI = Can’t produce ADH (responds to Challenge). Nephrogenic DI = No response to ADH.