Complex sentence: During a necropsy, a diagnosis is usually made very quickly, as the affected muscle is usually mottled with black patches, which are dead tissue, killed by the toxins that the bacteria release when they infect live tissue.
Split into simpler sentences: During a necropsy, a diagnosis is usually made very quickly, as the affected muscle is generally mottled with black patches. These patches represent dead tissue, killed by the toxins that the bacteria release when they infect a live tissue.