As a med student, you have to make up ridiculous methods for memorizing certain facts. Bethanechol and Methacholine are both muscarinic receptor agonists, but Bethanechol only affects Muscarinic receptors, while Methacholine activates both muscarinic and nicotinic receptors.
To memorize this, think of AcetylCHOLINE. MethaCHOLINE has the whole "CHOLINE" in it, therefore, like Acetylcholine, it activates both M and N receptors. BethaneCHOL has only half what Acetylcholine has, therefore it only activates M receptors.
Now you won't ever again forget what these 2 drugs do!