HCN causes-

Ans. is `a' i.e., Histotoxic anoxia by inhibiting cytochrome oxidase Cyanide inhibits cellular respirationby inhibiting cytochrome oxidase, hence oxygen cannot be utilized by cells. Death is due to histotoxic (cytotoxic) anoxia. HCN is less effective if kept too long or if patient has achlorhydria. Fatal dose of HCN is 50-60 mg (or 300 mg NaCN/KCN). Potassium ferrocyanide is nonpoisonous salt of cyanide.