Correct Answer: D. Prazosin
Scorpion venom (particularly from Indian red scorpion, Mesobuthus tamulus) acts as a sympathomimetic agent, causing massive catecholamine release and resulting in a hyperadrenergic state with severe hypertension, tachycardia, pulmonary edema, and cardiogenic shock. The venom triggers uncontrolled α- and β-adrenergic stimulation. Prazosin, a selective α1-adrenergic antagonist, directly blocks the peripheral α1-receptors responsible for vasoconstriction and hypertension, making it the physiological antidote. It counteracts the venom's sympathomimetic effects without blocking β-receptors (which would worsen pulmonary edema and cardiogenic shock). In Indian clinical practice, prazosin is the DOC for managing the hypertensive phase of scorpion envenomation, often combined with IV fluids and supportive care. The term "physiological antidote" specifically refers to a drug that produces opposite physiological effects to the toxin—prazosin does exactly this by opposing α-adrenergic overstimulation. Non-selective β-blockers or combined α/β-blockers are avoided in the acute phase because unopposed α-stimulation would be catastrophic.
Why the other options are wrong
A. Physostigmine — Physostigmine is an anticholinesterase that increases acetylcholine levels, enhancing parasympathetic activity. While it may theoretically oppose some sympathomimetic effects, it is NOT the physiological antidote to scorpion venom. It does not directly block α-adrenergic receptors and can cause cholinergic toxicity (bronchospasm, bradycardia, seizures) in the setting of venom-induced catecholamine surge. NBE trap: students may confuse anticholinesterases with sympatholytic agents. B. Atropine — Atropine is an antimuscarinic agent that blocks parasympathetic effects. Scorpion venom causes sympathomimetic (not parasympathomimetic) toxicity, so atropine is not indicated as the primary antidote. While atropine may be used adjunctively for bradycardia if it occurs, it does not address the core α-adrenergic overstimulation causing hypertension and shock. This is a classic distractor for students who confuse sympathomimetic with parasympathomimetic toxidromes. C. Norepinephrine — Norepinephrine is a sympathomimetic agent (α- and β-agonist) that would WORSEN scorpion venom toxicity by further increasing catecholamine-mediated vasoconstriction, hypertension, and cardiac arrhythmias. It is contraindicated in scorpion envenomation. NBE trap: students may confuse 'antidote' with 'treatment' and think a sympathomimetic supports failing circulation, missing that the problem is excessive sympathomimetic activity, not deficiency.
High-Yield Facts
- Prazosin is the physiological antidote to scorpion venom because it selectively blocks α1-adrenergic receptors and opposes the venom's sympathomimetic effects.
- Indian red scorpion (Mesobuthus tamulus) venom causes hyperadrenergic toxidrome: hypertension, tachycardia, pulmonary edema, and cardiogenic shock due to massive catecholamine release.
- Physiological antidote = a drug producing opposite effects to the toxin; prazosin opposes α-adrenergic overstimulation, making it the true physiological antidote.
- Non-selective β-blockers are contraindicated in acute scorpion envenomation because unopposed α-stimulation worsens hypertension and coronary vasoconstriction.
- Management of scorpion envenomation in India includes prazosin for hypertension, IV fluids, oxygen, and supportive care; antivenom use is controversial and not standard.
Mnemonics
VENOM = Sympathomimetic → Prazosin (α-blocker) Venom causes catecholamine surge (sympathomimetic) → need α-blocker (prazosin) to oppose it. Not anticholinergic (atropine) or more sympathomimetic (norepinephrine). Prazosin = Physiological Antidote (opposite effect) Venom = α-stimulation (vasoconstriction, ↑BP) → Prazosin = α-blockade (vasodilation, ↓BP). Direct opposition = physiological antidote.
NBE Trap
NBE pairs scorpion venom with 'antidote' to lure students into choosing anticholinergic (atropine) or sympathomimetic (norepinephrine) agents, confusing toxidrome classification. The key discriminator is recognizing that scorpion venom causes sympathomimetic (not parasympathomimetic) toxicity, requiring an α-blocker, not an antimuscarinic or sympathomimetic.
Clinical Pearl
In Indian emergency departments, a patient with scorpion sting presenting with severe hypertension (often >180/120 mmHg), pulmonary crackles, and tachycardia is immediately recognized as hyperadrenergic envenomation. Prazosin IV/IM rapidly reverses the hypertensive crisis by blocking α1-receptors, preventing the fatal progression to cardiogenic shock—this is why it is the physiological antidote, not merely a supportive agent.
_Reference: Forensic Medicine & Toxicology (Parikh) Ch. 8 (Toxicology); KD Tripathi Pharmacology Ch. 10 (Adrenergic Antagonists)_