Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 641: Which short-acting anticholinesterase drug is primarily used for the diagnosis of myasthenia gravis?

A. Edrophonium (Correct Answer)
B. Demecarium
C. Dyflos
D. Echothiophate

Explanation: ***Edrophonium*** - This **short-acting anticholinesterase** reversibly inhibits acetylcholinesterase, leading to a temporary increase in acetylcholine at the neuromuscular junction. - The **Tensilon test** uses edrophonium to rapidly assess for improved muscle strength in patients suspected of having myasthenia gravis. *Demecarium* - This is a **long-acting cholinesterase inhibitor** and is not typically used for the fast diagnostic test of myasthenia gravis due to its extended duration of action. - It was historically used in the treatment of glaucoma, not for acute diagnostic purposes in neuromuscular disorders. *Dyflos* - Also known as **diisopropyl fluorophosphate (DFP)**, Dyflos is an **irreversible organophosphorus cholinesterase inhibitor**. - Its long-lasting and toxic effects make it unsuitable and dangerous for diagnostic testing of myasthenia gravis. *Echothiophate* - This is another **long-acting, irreversible cholinesterase inhibitor** primarily used in the treatment of glaucoma. - Its irreversible nature and prolonged effects are not suitable for the quick, reversible diagnostic test needed for myasthenia gravis.

Question 642: What do organophosphates inhibit in the human body?

A. The esteratic site of acetylcholinesterase (AchE) (Correct Answer)
B. The anionic site of acetylcholinesterase (AchE)
C. Acetylcholine (Ach)
D. None of the options

Explanation: ***The esteratic site of acetylcholinesterase (AchE)*** - Organophosphates are **irreversible inhibitors** that bind covalently to the **esteratic site** (serine hydroxyl group) of acetylcholinesterase. - This binding prevents the enzyme from hydrolyzing **acetylcholine**, leading to its accumulation in the synaptic cleft. - This covalent phosphorylation is the hallmark of organophosphate poisoning. *The anionic site of acetylcholinesterase (AchE)* - The **anionic site** of acetylcholinesterase binds to the **quaternary ammonium group** of acetylcholine, facilitating its orientation for hydrolysis at the esteratic site. - While important for enzyme function, organophosphates do not primarily act by inhibiting this site. - Reversible cholinesterase inhibitors (like edrophonium) interact with both sites but do not form covalent bonds. *Acetylcholine (Ach)* - Organophosphates do not directly inhibit **acetylcholine** itself. - Instead, they inhibit the **enzyme that breaks down acetylcholine** (acetylcholinesterase). - The accumulation of acetylcholine is a **consequence** of enzyme inhibition, not the primary target of organophosphate action. *None of the options* - This option is incorrect because the first option accurately describes the specific site of organophosphate inhibition on acetylcholinesterase.

Question 643: Which drug is generally considered the first-line treatment for pheochromocytoma?

A. Prazosin
B. Phentolamine
C. Phenoxybenzamine (Correct Answer)
D. Propranolol

Explanation: ***Phenoxybenzamine (Correct Answer)*** - **Phenoxybenzamine** is a **non-competitive, non-selective alpha-adrenergic blocker** and is considered the **first-line treatment for pheochromocytoma** to prevent hypertensive crises during surgery. - It **irreversibly blocks** both alpha-1 and alpha-2 adrenergic receptors, leading to **vasodilation** and prevention of catecholamine-induced vasoconstriction. - Its **long duration of action** and irreversible binding make it ideal for pre-operative preparation. *Propranolol (Incorrect)* - **Propranolol** is a **beta-blocker** that should **never be used first** in pheochromocytoma as primary treatment. - Blocking beta-2 receptors can lead to **unopposed alpha-adrenergic vasoconstriction**, potentially causing a **hypertensive crisis**. - Beta-blockers are only introduced **after adequate alpha-blockade** to manage tachycardia and arrhythmias. *Prazosin (Incorrect)* - **Prazosin** is a **selective alpha-1 blocker** and competitive antagonist that may not provide sufficient protection against the massive surge of catecholamines during surgery. - While it can lower blood pressure, its effect is **reversible** and **selective**, making it less reliable than non-selective, irreversible blockers in this critical setting. *Phentolamine (Incorrect)* - **Phentolamine** is a **short-acting, non-selective alpha blocker** typically used for acute management of **hypertensive crises** (e.g., during surgery or in critical care) rather than for pre-operative preparation. - Its **shorter duration of action** requires frequent dosing and makes it less suitable for the sustained alpha-blockade needed in the days leading up to pheochromocytoma surgery.

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

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Adrenergic Agonists

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Adrenergic Antagonists

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Ganglionic Agents

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Neuromuscular Blocking Agents

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Autonomic Drugs in Ophthalmology

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Autonomic Drugs in Cardiovascular Disease

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Autonomic Drugs in Respiratory Disease

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Autonomic Drugs in Urological Disorders

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