A patient with nephrotic syndrome has severe hypoalbuminaemia (serum albumin 1.5 g/dL). Despite massive peripheral oedema, she is hypotensive. The physio-pathological explanation is:

• A Overfill theory: primary renal Na retention causes intravascular expansion
• B Lymphatic blockade preventing return of interstitial fluid
• C Underfill theory: low oncotic pressure depletes intravascular volume ✓
• D Hyperaldosteronism causing K+ wasting and vascular dysfunction

Explanation

The 'underfill' theory applies when hypoalbuminaemia is severe: reduced plasma oncotic pressure causes fluid to shift from capillaries into interstitium, reducing effective circulating volume and leading to hypotension. This triggers secondary activation of the renin-angiotensin-aldosterone system and ADH, causing Na and water retention which worsens oedema. The overfill theory applies in less severe cases or primary renal disease with preserved oncotic pressure.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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