Postmortem hypostasis (livor mortis) is described as 'non-shifting' after approximately 8–12 hours. Which biochemical process primarily accounts for this fixation?

• A Rigor mortis of the vessel wall smooth muscle trapping blood in dependent vessels
• B Denaturation of haemoglobin by lactic acid converting it to methaemoglobin, which diffuses into tissues ✓
• C Complete depletion of ATP causing irreversible vascular smooth muscle contraction
• D Fibrin clot formation sealing the capillaries and preventing blood movement

Explanation

Initially, lividity shifts freely because erythrocytes can redistribute with body position change. Over time, haemoglobin is progressively converted to methaemoglobin (and eventually haematin/porphyrin derivatives) by the acid environment of autolysis. These breakdown pigments diffuse out of the vessels and stain the adjacent tissue, producing fixed (non-shifting) livor. ATP depletion causes rigor in striated muscle, not in vessel walls. Fibrin clotting may contribute marginally but is not the primary mechanism of fixation.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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