A non-vital tooth discolored from a year-old traumatic injury. The discoloration is MOST caused by:

Correct answer: A. Breakdown of red blood cells (RBCs) and hemoglobin degradation products (hemosiderin, hemin) infiltrating dentinal tubules

• A. Correct. After pulp trauma with intrapulpal hemorrhage, RBCs break down. Hemoglobin is degraded to hemosiderin and iron sulfide compounds (from bacterial H2S), which are small molecules that penetrate dentinal tubules and cause grey/dark discoloration of the crown. This is the primary mechanism of post-traumatic tooth discoloration.
• B. WBC breakdown does not produce the pigmented compounds that cause tooth discoloration.
• C. Bacterial endotoxins contribute to pulp necrosis and periapical disease but are not the primary cause of crown discoloration.
• D. Enamel thinning from attrition can expose yellowish dentin but this is not a post-traumatic mechanism.