NBME 18 Answers : Block 1 : #3 = 5 yo boy, history of motion sickness

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submitted by iguzman2(1),

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erAnt' 1M psrocrete unodf ni teh nirba adn rea sorlsbeenpi fro mitono esksnc?is

m-ice M1 receptors are for sure the major muscarinic receptor type found in the CNS, but M3 receptors are the muscarinic subtype involved in vomiting controlled by the CNS. This is definitely a random fact, but I think they also wanted you to eliminate all other options. Targeting the sympathetic system (options A and B) won't make a difference. NMDA receptors are a major receptor throughout the CNS, but they're not a target of antihistamines, and neither are serotonin receptors. We know that antihistamines target histamine and muscarinic receptors, but the H2 histamine receptor is responsible for gastrin secretion in the stomach, so the answer must be antagonism of M3 receptors. +4

dorsal_vein ^ First generation antihistamines definitely antagonize serotonin receptors within the CNS, which can cause weight gain and increase appetite. However, this plays little role in motion sickness. +5

mumenrider4ever That is confusing because scopolamine (anti-muscarinic used to treat motion sickness) is an M1 receptor antagonist +1

pelparente So according to amboss scopolamine is a nonspecific antiemetic. I think Sketchy probably just confused everyone. https://www.amboss.com/us/knowledge/Antiemetics +1