**Question:** 1% w/v of methylene blue is injected intravenously into the tissue to reverse the toxic effects of

A. Cyanide poisoning
B. Acetaminophen poisoning
C. Organophosphorus pesticide poisoning
D. Sulfonamide poisoning

**Core Concept:**

Methylene blue is a dye and a medication that has various uses, including reversal of poisoning caused by certain substances due to its mechanism of action. Methylene blue is a competitive inhibitor of mitochondrial cytochrome c oxidase, which is an enzyme involved in cellular respiration. This inhibition leads to a shift in cellular energy production from oxidative phosphorylation to glycolysis, thus reducing the toxic effects of substances that inhibit cytochrome c oxidase.

**Why the Correct Answer is Right:**

Cyanide poisoning is caused by hydrogen cyanide (HCN) accumulation, which inhibits the enzyme cytochrome c oxidase in the electron transport chain (ETC) of mitochondria. This inhibition leads to a decrease in cellular energy production, impaired oxidative phosphorylation, and accumulation of toxic substances. Methylene blue's inhibition of cytochrome c oxidase competitively competes with HCN for binding to the enzyme, effectively counteracting the toxic effects of cyanide poisoning.

**Why Each Wrong Option is Incorrect:**

- Acetaminophen poisoning (option B): Methylene blue does not have a role in treating acetaminophen (paracetamol) poisoning, as acetaminophen poisoning primarily causes liver damage through the formation of reactive metabolites, which bind to proteins and cause cellular injury. N-acetylcysteine is the standard treatment for acetaminophen poisoning.

- Organophosphorus pesticide poisoning (option C): Organophosphates and carbamates are inhibitors of acetylcholinesterase, which leads to an accumulation of acetylcholine (ACh) at post-synaptic receptors. This results in overstimulation of muscarinic and nicotinic receptors, causing a range of symptoms such as salivation, bronchorrhea, bronchospasm, and seizures. Methylene blue does not have a role in treating organophosphorus poisoning.

- Sulfonamide poisoning (option D): Sulfonamides are antibiotics that work by inhibiting bacterial dihydropteroate synthetase, an enzyme involved in the synthesis of folic acid, leading to folic acid deficiency. However, this does not typically cause significant clinical symptoms or require methylene blue treatment.

**Clinical Pearl:**

Methylene blue is primarily used as a dye for staining and as a medication for treatment of methemoglobinemia and cyanide poisoning. In methemoglobinemia, methylene blue acts as a reductase that reduces methemoglobin to hemoglobin, allowing oxygen to be transported effectively. In cyanide poisoning, methylene blue competes with cyanide for binding to methemoglobin, preventing its binding and reversing its toxic effects. In both cases, methylene blue competes with the toxic substance for binding to the target site, thereby reducing its toxic effects.

For cyanide poisoning: