A 60-year-old man dies 5 days after a large anterior MI. Autopsy reveals a free wall rupture of the left ventricle with hemopericardium. Which histological process was occurring in the necrotic myocardium that predisposed to rupture at this specific time point?

• A Granulation tissue formation with new capillaries replacing necrotic myocardium
• B Maximal neutrophilic infiltration with collagenase release causing myocardial softening (yellow softening phase) ✓
• C Fibroblast proliferation and dense fibrous scar formation reducing mechanical vulnerability
• D Contraction band necrosis with reperfusion calcium overload causing sarcomere contracture

Explanation

Free wall rupture peaks at days 3–7 post-MI, corresponding to the phase of maximal neutrophilic infiltration and macrophage phagocytosis of necrotic debris. Neutrophils and macrophages release matrix metalloproteinases (MMPs) and elastase, degrading extracellular matrix collagen and causing 'yellow softening' of the myocardium. This enzymatic digestion combined with the mechanical stress of systolic contraction on the softened necrotic zone predisposes to rupture. Granulation tissue replaces necrotic tissue from day 7–10 onward, and mature scar (fibroblast/collagen) forms by 6–8 weeks, progressively increasing tensile strength.

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

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Written and medically reviewed by the StethoPrep medical team.