A 30-year-old woman with nephrotic syndrome (serum albumin 1.8 g/dL) develops bilateral pitting pedal edema. Which Starling force change is primarily responsible?

• A Elevated capillary hydrostatic pressure
• B Reduced capillary hydrostatic pressure
• C Elevated interstitial oncotic pressure
• D Reduced plasma oncotic pressure ✓

Explanation

Filtration across capillaries is governed by the balance of capillary hydrostatic pressure pushing fluid out and plasma colloid osmotic (oncotic) pressure pulling fluid in. Heavy proteinuria lowers serum albumin, so plasma oncotic pressure falls and net filtration exceeds lymphatic drainage capacity, producing generalized edema. Venous obstruction or heart failure would raise capillary hydrostatic pressure instead. Inflammation raises interstitial oncotic pressure by increasing capillary permeability to protein, which does not occur here.

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

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