A patient with AA amyloidosis secondary to rheumatoid arthritis has the amyloid protein derived from serum amyloid A (SAA). Which organ shows the most characteristic 'lardaceous' gross appearance on autopsy and demonstrates positive staining with Congo red showing apple-green birefringence?

• A Kidney — preferential glomerular deposition causing waxy pale cortex
• B Spleen — either diffuse 'sago spleen' (red pulp) or 'lardaceous spleen' (white pulp) ✓
• C Liver — with 'lardaceous liver' (waxy, pale, enlarged) and perivascular sinusoidal deposition
• D Heart — with 'stiff heart syndrome' and ventricular wall thickening

Explanation

In AA amyloidosis, the spleen shows two classic gross patterns: 'sago spleen' (deposition in splenic follicles/white pulp giving grey translucent nodules resembling sago grains) and 'lardaceous spleen' (diffuse deposition in red pulp cords giving a waxy homogeneous appearance). Both show apple-green birefringence with Congo red under polarized light. While renal involvement is clinically the most important (leading to nephrotic syndrome), the classic gross pathological teaching descriptor is for the spleen.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.