INI-CET 2015 — Pharmacology

Q: Methacholine acts primarily at which receptor?

M3. ***M3*** - **Methacholine** is a non-selective muscarinic agonist that acts at all muscarinic receptors, but its **primary clinically relevant effects**, particularly in the airways (e.g., bronchoconstriction in asthma challenge tests), are mediated through **M3 receptors**. - **M3 receptors** are Gq-coupled and lead to **smooth muscle contraction**, glandular secretion, and vasodilation via nitric oxide release. *M1* - **M1 receptors** are primarily found in the central nervous system and autonomic ganglia, playing a role in **neuronal excitation** and **autonomic transmission**. - While methacholine can activate M1 receptors, this is not its predominant site of action for clinical uses. *M2* - **M2 receptors** are found in the heart and are Gi-coupled, leading to a **decrease in heart rate** and contractility. - While methacholine acts on M2 receptors, its most notable effects (like bronchoconstriction) are not primarily mediated through M2. *M4* - **M4 receptors** are primarily located in the central nervous system, where they modulate neurotransmitter release and contribute to **motor control** and cognition. - Methacholine has some affinity for M4, but it is not the primary receptor responsible for its clinically relevant effects. [Practice more Pharmacology PYQs on OnCourse]

Q: A 60-year-old hypertension patient presents with palpitations and pre-syncope. ECG was done. All drugs can be used for management of this condition except:

Adenosine. ***Adenosine*** - The ECG shows **atrial fibrillation with rapid ventricular response** and evidence of **pre-excitation** (short PR interval, delta wave), suggesting **Wolff-Parkinson-White (WPW) syndrome** complicated by atrial fibrillation. - Adenosine is **absolutely contraindicated** in AF with WPW syndrome because it causes profound AV nodal blockade, forcing all conduction down the accessory pathway, which can precipitate **life-threatening ventricular fibrillation**. - Among AV nodal blockers, **adenosine poses the greatest risk** due to its rapid, complete AV nodal blockade and is the classic teaching example of drugs to avoid in this condition. *Diltiazem* - **Calcium channel blockers** like diltiazem should generally be avoided in AF with WPW because they block the AV node and can worsen conduction via the accessory pathway, potentially leading to VF. - However, their effect is less abrupt than adenosine, making them somewhat less immediately dangerous. *Esmolol* - **Beta-blockers** like esmolol should also be avoided in AF with WPW syndrome because they block the AV node and can increase conduction through the accessory pathway. - Like calcium channel blockers, the risk is real but less catastrophic than with adenosine. *Amiodarone* - Amiodarone is a **Class III antiarrhythmic** that **can be safely used** for rhythm control in AF with WPW syndrome. - Unlike pure AV nodal blockers, it affects **both the AV node and accessory pathway**, helping to slow conduction through both routes without preferentially shunting impulses down the dangerous accessory pathway. - It is considered appropriate for pharmacological management of this condition. [Practice more Pharmacology PYQs on OnCourse]

Q: What is the drug of choice for pregnancy-induced hypertension?

Alpha-methyldopa. **Explanation:** **Alpha-methyldopa** is the traditional drug of choice for chronic hypertension in pregnancy. It is a centrally acting alpha-2 adrenergic agonist that reduces sympathetic outflow. Its preference in pregnancy stems from its long-standing safety profile and the absence of long-term adverse effects on fetal development or neonatal hemodynamics. **Analysis of Options:** * **Atenolol (Option A):** Beta-blockers, particularly Atenolol, are generally avoided in early pregnancy as they are associated with **fetal growth restriction (IUGR)** and placental complications. * **Nitroprusside (Option B):** It is contraindicated in pregnancy (except in extreme emergencies) due to the risk of **fetal cyanide poisoning**. * **Enalapril (Option C):** ACE inhibitors and ARBs are strictly **contraindicated** in pregnancy. They are teratogenic and can cause fetal renal dysgenesis, oligohydramnios, and skull hypoplasia. **High-Yield Clinical Pearls for NEET-PG:** * **First-line agents:** While Methyldopa is the classic answer, current guidelines (like ACOG) also recommend **Labetalol** (often preferred for its faster onset) and **Nifedipine** (long-acting) as first-line options. * **Acute Hypertensive Crisis in Pregnancy:** The drugs of choice are **IV Labetalol** or **IV Hydralazine**. * **Preeclampsia Prophylaxis:** Low-dose **Aspirin** (started before 16 weeks) is used in high-risk patients. * **Magnesium Sulfate ($MgSO_4$):** This is the drug of choice for preventing and treating seizures in **Eclampsia**, not for blood pressure control. [Practice more Pharmacology PYQs on OnCourse]

3 Previous Year Questions with Answers & Explanations

Questions

All Subjects Anesthesiology (2)Community Medicine (2)Dermatology (2)ENT (1)Forensic Medicine (1)Internal Medicine (1)Internal Medicine (1)Microbiology (5)Obstetrics and Gynecology (4)Pathology (4)Pathology (3)Pediatrics (2)Pharmacology (3)Radiology (4)Surgery (1)

Methacholine acts primarily at which receptor?

A 60-year-old hypertension patient presents with palpitations and pre-syncope. ECG was done. All drugs can be used for management of this condition except:

Q3Easy

What is the drug of choice for pregnancy-induced hypertension?