This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Anatomy
A patient on nitroprusside therapy developed cyanide toxicity. Sodium nitrite was administered i.v. to combat this poisoning. Beneficial effect of sodiumnitrite in this case is dependent on: 
Select and explain. 
A. Direct chelation of cyanide with sodium nitrite, B. Vasodilatation caused by sodium nitrite, C. Conversion of hemoglobin to methemoglobin by sodium nitrite, D. Facilitation of cyanocobalamin formation by sodium nitrite
C: Conversion of hemoglobin to methemoglobin by sodium nitrite
Answer: C: Conversion of hemoglobin to methemoglobin by sodium nitrite 
Explanation: (Ref: KDT 6/e p527) Cyanide has very high affinity for the enzymes of respiratory chain paicularly cytochrome oxidase. Methemoglobin possesses even higher affinity for cyanides. It can free the enzymes due to competitive inhibition for cyanide. Nitrates act by reducing hemoglobin to mithenoglobin, which removes cyanide from the enzymes.