All of the following drugs cause tachycardia except?
**Core Concept**
The question is testing knowledge of pharmacological agents that affect the autonomic nervous system, particularly those that cause tachycardia. Tachycardia is an increased heart rate, typically above 100 beats per minute, which can be caused by various factors including drugs, anxiety, fever, and cardiac disease.
**Why the Correct Answer is Right**
To determine the correct answer, we need to consider the pharmacological effects of each option on the heart rate. Most of the drugs listed are beta-adrenergic agonists or have sympathomimetic properties, which stimulate the heart and increase heart rate. However, one of the options is a drug that has a parasympathomimetic effect, which would decrease heart rate rather than increase it.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a beta-adrenergic agonist, such as isoproterenol, which increases heart rate and contractility by stimulating beta-1 receptors in the heart. It is therefore a correct cause of tachycardia.
**Option B:** This option is likely a sympathomimetic drug, such as ephedrine, which increases heart rate and blood pressure by stimulating alpha and beta-adrenergic receptors. It is therefore a correct cause of tachycardia.
**Option C:** This option is likely a drug that blocks the action of acetylcholine at muscarinic receptors, such as atropine, which decreases parasympathetic tone and increases heart rate. It is therefore a correct cause of tachycardia.
**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the drugs that cause tachycardia is "BEAT": Beta-agonists, Ephedrine, Atropine, and other sympathomimetics can all increase heart rate.
**Correct Answer:** D.