Which one of the following is the best definition of gangrene?

The answer is A. In gangrene, tissue which is dead is digested by bacteria which are incapable of invading and multiplying in living tissue (saprophytes). Gas production (C) may be present in some forms of gangrene particularly when caused by the anaerobic Clostridia.

Necrosis of tissue is an essential prerequisite for gangrene, but it may be caused by ischaemia (E), i.e., secondary gangrene or by bacterial toxins (D), i.e., primary gangrene.