This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
Causes of cynosls- 
Select and explain. 
A. TOF, B. PDA, C. Tricuspid atresia, D. Eisonmenger's complex
A: TOF
Answer: A: TOF 
Explanation: Tetralogy of Fallot causes low oxygen levels in the blood. This leads to cyanosis (a bluish-purple color to the skin). The classic form includes four defects of the hea and its major blood vessels: Ventricular septal defect (hole between the right and left ventricles) Ref Davidson 23rd edition pg 455