USMLE Step 1 Quiz: Autonomic Pharmacology

What this quiz covers

This quiz focuses on Autonomic Pharmacology, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Edrophonium
2. Physostigmine
3. Pralidoxime (correct answer)
4. Neostigmine

Explanation: Organophosphates are irreversible acetylcholinesterase inhibitors. Atropine is a muscarinic antagonist that competitively blocks the effects of excess acetylcholine at muscarinic receptors, treating symptoms like bradycardia, salivation, and bronchospasm. However, it has no effect on nicotinic receptors (responsible for muscle fasciculations and weakness). Pralidoxime is a cholinesterase reactivator that removes the organophosphate from the enzyme, restoring its function. It must be given early, before the bond 'ages'.

Question 2

1. Muscarinic cholinergic receptors (correct answer)
2. Nicotinic cholinergic receptors
3. Alpha-adrenergic receptors
4. Beta-adrenergic receptors

Explanation: The patient is presenting with classic anticholinergic (antimuscarinic) toxicity, often remembered by the mnemonic 'hot as a hare, blind as a bat, dry as a bone, red as a beet, mad as a hatter.' The toxins in Atropa belladonna, such as atropine and scopolamine, are potent competitive antagonists of muscarinic cholinergic receptors, leading to inhibition of parasympathetic functions.

Question 3

1. Diarrhea
2. Bradycardia
3. Excessive tearing
4. Dry mouth (correct answer)

Explanation: Oxybutynin is an antimuscarinic agent that blocks M3 receptors on the detrusor muscle, causing it to relax and increasing bladder capacity. However, it also blocks muscarinic receptors elsewhere in the body, leading to classic anticholinergic side effects. These include dry mouth (xerostomia) due to blockade of M3 receptors on salivary glands, constipation, blurred vision, and confusion, especially in the elderly.

Question 4

1. Alpha-1 adrenergic (correct answer)
2. Beta-1 adrenergic
3. Beta-2 adrenergic
4. Dopamine D1

Explanation: Phenylephrine is a selective alpha-1 adrenergic agonist. Activation of alpha-1 receptors on vascular smooth muscle causes potent vasoconstriction, which increases systemic vascular resistance (SVR) and, consequently, blood pressure. It has minimal effect on beta receptors, so it does not directly increase heart rate or contractility and may cause a reflex bradycardia.

Question 5

1. Blockade of alpha-1 receptors in peripheral arterioles
2. Stimulation of central presynaptic alpha-2 receptors (correct answer)
3. Blockade of beta-1 receptors in the juxtaglomerular apparatus
4. Direct relaxation of vascular smooth muscle via nitric oxide

Explanation: Clonidine is a centrally acting alpha-2 adrenergic agonist. It stimulates alpha-2 receptors in the brainstem (specifically the nucleus tractus solitarii), which inhibits presynaptic release of norepinephrine. This leads to a decrease in central sympathetic outflow, reducing heart rate, cardiac output, and peripheral vascular resistance, thereby lowering blood pressure.

Question 6

1. Systemic vascular resistance
2. Myocardial contractility (correct answer)
3. Pulmonary capillary wedge pressure
4. Afterload

Explanation: Dobutamine is a synthetic catecholamine that is primarily a beta-1 adrenergic agonist. Stimulation of beta-1 receptors in the heart leads to a significant increase in myocardial contractility (positive inotropy) and, to a lesser extent, heart rate. This improves cardiac output and organ perfusion in cardiogenic shock. Dobutamine also has weak beta-2 agonist activity, which can cause mild vasodilation and a decrease in systemic vascular resistance (afterload).

Question 7

1. Beta-2 receptors in skeletal muscle and cardiac tissue (correct answer)
2. Alpha-1 receptors in peripheral vasculature
3. Muscarinic M2 receptors in the sinoatrial node
4. Beta-1 receptors in the kidney

Explanation: Albuterol is a short-acting beta-2 selective adrenergic agonist used for bronchodilation. However, at higher doses or with frequent use, its selectivity decreases. The tremor (shaky hands) results from stimulation of beta-2 receptors in skeletal muscle. The tachycardia (heart racing) occurs due to stimulation of both beta-2 receptors present in cardiac tissue and some cross-reactivity with cardiac beta-1 receptors at higher doses.

Question 8

1. Reflex tachycardia
2. First-dose orthostatic hypotension (correct answer)
3. Central nervous system sedation
4. Drug-induced bradycardia

Explanation: Prazosin is an alpha-1 adrenergic antagonist. It blocks alpha-1 receptors on both arterial and venous smooth muscle, causing vasodilation and a decrease in blood pressure. A well-known adverse effect, particularly with the first dose or after a dose increase, is orthostatic hypotension. This occurs because the blockade of venous alpha-1 receptors prevents the normal vasoconstrictor response needed to maintain blood pressure upon standing.

Question 9

1. Beta-1 and Beta-2 only
2. Beta-1 only
3. Alpha-1, Beta-1, and Beta-2 (correct answer)
4. Muscarinic M2 and Beta-1

Explanation: Carvedilol (and labetalol) are non-selective beta-blockers that also have alpha-1 blocking properties. In heart failure, the beta-1 blockade reduces the harmful effects of chronic sympathetic stimulation on the heart (e.g., remodeling, apoptosis). The alpha-1 blockade causes peripheral vasodilation, which reduces both preload and afterload, further decreasing the workload on the failing heart.

Question 10

1. Serotonin 5-HT3 receptors
2. Histamine H1 receptors
3. Dopamine D2 receptors
4. Muscarinic cholinergic receptors (correct answer)

Explanation: Scopolamine is a centrally acting antimuscarinic agent. It is particularly effective for motion sickness because it blocks muscarinic receptors in the vestibular system and the brainstem vomiting center. The side effects of drowsiness and dry mouth are due to its central and peripheral anticholinergic properties, respectively.

Question 11

1. Competitive antagonist
2. Full agonist
3. Partial agonist (correct answer)
4. Non-competitive antagonist

Explanation: Varenicline is a partial agonist at the α4β2 nicotinic acetylcholine receptor subtype. As a partial agonist, it provides a moderate, sustained level of dopamine release in the mesolimbic pathway, which helps to reduce cravings and withdrawal symptoms (agonist activity). At the same time, it occupies the receptor and prevents nicotine from binding, thereby blocking the rewarding effects of smoking (antagonist activity).

Question 12

1. Antagonism of muscarinic M3 receptors on the detrusor muscle
2. Stimulation of beta-3 adrenergic receptors on the detrusor muscle (correct answer)
3. Blockade of alpha-1 adrenergic receptors in the bladder neck
4. Inhibition of phosphodiesterase type 5 in bladder smooth muscle

Explanation: Mirabegron is a beta-3 adrenergic agonist. Stimulation of beta-3 receptors on the detrusor muscle leads to smooth muscle relaxation, which increases the bladder's capacity to store urine and reduces symptoms of urgency and frequency. This mechanism is distinct from antimuscarinic drugs like oxybutynin, which block M3 receptors to prevent involuntary bladder contractions. Because mirabegron does not affect muscarinic receptors, it avoids the typical anticholinergic side effects.

Question 13

1. Direct stimulation of muscarinic receptors on the detrusor muscle (correct answer)
2. Inhibition of acetylcholinesterase at the neuromuscular junction
3. Antagonism of muscarinic receptors in the bladder trigone
4. Direct stimulation of nicotinic receptors at the autonomic ganglia

Explanation: Bethanechol is a direct-acting cholinomimetic that selectively stimulates muscarinic receptors, particularly M3 receptors on the detrusor muscle of the bladder and on smooth muscle of the GI tract. This stimulation leads to detrusor muscle contraction and relaxation of the trigone and sphincter, promoting urination. It is used to treat non-obstructive urinary retention.

Question 14

1. Alpha-1A (correct answer)
2. Alpha-1B
3. Alpha-1D
4. Alpha-2A

Explanation: Alpha-1 receptors have subtypes. Alpha-1A receptors are predominantly found in the smooth muscle of the prostate and bladder neck, and their blockade by tamsulosin relaxes these tissues, improving urinary flow in BPH. Alpha-1B receptors are primarily found in vascular smooth muscle. Prazosin is non-selective for these subtypes and blocks both, leading to significant vasodilation and a risk of orthostatic hypotension. Tamsulosin's selectivity for Alpha-1A makes it uroselective with fewer vascular side effects.

Question 15

1. Stimulation of beta-2 adrenergic receptors, leading to bronchodilation
2. Antagonism of muscarinic receptors, leading to bronchodilation (correct answer)
3. Inhibition of phosphodiesterase, increasing intracellular cAMP
4. Blockade of histamine H1 receptors on bronchial smooth muscle

Explanation: Ipratropium is a short-acting muscarinic antagonist (SAMA). It competitively blocks muscarinic receptors (primarily M3) on airway smooth muscle, preventing acetylcholine-induced bronchoconstriction. This results in bronchodilation. It is a quaternary amine, so it does not readily cross the blood-brain barrier and has minimal systemic side effects.

Question 16

1. Hypertensive crisis
2. Bronchoconstriction (correct answer)
3. Diuresis
4. Coronary vasospasm

Explanation: Propranolol is a non-selective beta-blocker, meaning it antagonizes both beta-1 and beta-2 adrenergic receptors. While the beta-1 blockade is beneficial for the heart post-MI, the blockade of beta-2 receptors in the lungs is problematic. Beta-2 receptors mediate bronchodilation. Blocking them can lead to bronchoconstriction, which can precipitate a severe asthma attack in susceptible individuals. Cardioselective beta-1 blockers (e.g., metoprolol, atenolol) are preferred in such patients.

Question 17

1. Beta-1 receptor-mediated increase in cardiac contractility
2. Beta-2 receptor-mediated bronchodilation
3. Alpha-1 receptor-mediated vasoconstriction (correct answer)
4. Beta-2 receptor-mediated inhibition of mast cell degranulation

Explanation: In anaphylactic shock, massive histamine release causes widespread vasodilation and capillary leak, leading to profound hypotension. Epinephrine is the treatment of choice because it addresses all aspects of anaphylaxis. However, the most critical life-saving effect for reversing shock is its potent alpha-1 agonist activity, which causes vasoconstriction, increases systemic vascular resistance, and raises blood pressure. The beta-2 effects (bronchodilation) and beta-1 effects (increased cardiac output) are also crucial but secondary to vasoconstriction for treating the shock itself.

Question 18

1. Decreasing the production of aqueous humor by the ciliary body
2. Contracting the ciliary muscle, which opens the trabecular meshwork (correct answer)
3. Relaxing the pupillary sphincter muscle, causing mydriasis
4. Blocking beta-adrenergic receptors on the ciliary epithelium

Explanation: Pilocarpine is a direct-acting muscarinic agonist. In the eye, it causes contraction of the ciliary muscle (a parasympathetic action). This contraction pulls on the trabecular meshwork, increasing the outflow of aqueous humor and thereby lowering intraocular pressure. It also causes miosis (pupillary constriction), which can further help open the angle in angle-closure glaucoma.

Question 19

1. It sensitizes postsynaptic nicotinic receptors to acetylcholine.
2. It increases the presynaptic release of acetylcholine vesicles.
3. It increases the duration of acetylcholine in the synaptic cleft. (correct answer)
4. It acts as a direct agonist at the nicotinic receptor.

Explanation: Pyridostigmine is a long-acting, reversible acetylcholinesterase (AChE) inhibitor. By inhibiting the AChE enzyme, it prevents the breakdown of acetylcholine in the neuromuscular junction. This increases the concentration and prolongs the half-life of acetylcholine in the synaptic cleft, allowing it to better compete for the reduced number of functional nicotinic receptors characteristic of myasthenia gravis, thereby improving neuromuscular transmission and muscle strength.

Question 20

1. Directly agonizes postsynaptic nicotinic receptors
2. Reversibly inhibits acetylcholinesterase (correct answer)
3. Blocks presynaptic voltage-gated calcium channels
4. Promotes the synthesis of acetylcholine in the presynaptic terminal

Explanation: Myasthenia gravis is treated with acetylcholinesterase (AChE) inhibitors, such as pyridostigmine or neostigmine. These drugs reversibly inhibit AChE, the enzyme that degrades acetylcholine in the synaptic cleft. This action increases the concentration and duration of action of acetylcholine at the neuromuscular junction, improving muscle strength. The side effects (cramps, diarrhea) are due to increased muscarinic stimulation in the GI tract.