This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Microbiology
In apoptosis, Apaf-1 is activated by release of which of the following substance from the mitochondria? 
Select and explain. 
A. Bcl - 2, B. Bax, C. Bcl - Xl, D. Cytochrome C
D: Cytochrome C
Answer: D: Cytochrome C 
Explanation: Apoptosis is induced by a cascade of molecular events all of the which culminate in the activation of caspases.Mitochondria contain proteins such as cytochrome c. Once released into the cytosol, cytochrome c binds to a protein called APAF-1 (apoptosis-activating factor-1), which forms a wheel-like hexamer that has been called the apoptosome. This complex activates caspase 9.