Question

Which of the following is used as antidote for cyanide poisoning in bovines?
A. Sodium EDTA
B. Sodium nitrite
C. Sodium thiosulphate
D. Dimercaprol
E. BAL
Choose the correct answer from the options given below:

• A. A and D only
• B. E only
• C. A, D and E only
• D. B and C only

Hint:

Cyanide Antidote Regimen: 1. Sodium Nitrite (induces MetHb trap) $+$ 2. Sodium Thiosulphate (substrate for Rhodanese $\rightarrow$ Thiocyanate excretion).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Dual antidotal synergy in hydrocyanic acid (HCN prussic acid) toxicity: methemoglobin induction by sodium nitrite followed by enzymatic rhodanese sulfuration by sodium thiosulfate.
Key Formula or Approach:
\[ \text{Hb (Fe}^{2+}\text{)} \xrightarrow{\text{Sodium Nitrite}} \text{MetHb (Fe}^{3+}\text{)} + \text{Cyanide} \longrightarrow \text{Cyanmethemoglobin (Non-toxic)} \]
\[ \text{Cyanmethemoglobin} + \text{Sodium Thiosulphate} \xrightarrow{\text{Rhodanese}} \text{Thiocyanate (Excreted)} + \text{MetHb} \]

Step 2: Detailed Explanation:
Standard two-step antidotal therapy for Bovine Cyanide (HCN Prussic Acid) Poisoning (e.g., from immature sorghum Johnson grass ingestion):
1. Step 1 (B. Sodium Nitrite, $1\% - 3\%$ i/v): Converts a fraction of hemoglobin ($ ext{Fe}^{2+}$) into Methemoglobin ($ ext{Fe}^{3+}$). Methemoglobin has a higher affinity for cyanide than mitochondrial cytochrome oxidase ($ ext{Fe}^{3+}$), pulling cyanide off cytochrome $a_3$ to form non-toxic cyanmethemoglobin and reactivating cellular respiration.
2. Step 2 (C. Sodium Thiosulphate, $20\% - 25\%$ i/v): Acts as a sulfur donor for the hepatic enzyme Rhodanese (thiosulfate sulfurtransferase), converting cyanmethemoglobin into non-toxic thiocyanate, which is harmlessly excreted in urine.
Thus, combination B and C only is the specific antidote.

Step 3: Final Answer:
Therefore, the antidote for cyanide poisoning is B and C only, corresponding to option (D).