Knowledge of the terms 'ylide' and 'betaine' is an index of fluency, so you might expect to see them at some point in an MCAT passage. When the MCAT writers are doing something like that, and you run into a bit of difficult terminology, you can usually figure out the meaning from the context. MCAT passages are meant to be hard, sometimes, to hold onto. An ylide is a neutral molecule with positive and negative charges on adjacent atoms. A betaine is a neutral molecule with a positively charged functional group and a negatively charged functional group. The Wittig reaction employs phosphorus ylides to convert aldehydes and ketones to alkenes. One of the intermediates of the reaction is a 'betaine', a form containing opposite formal charges that are not necessarilly adjacent.