People who misunderstand the optimal purchase rule argue that if a consumer buys so much of a good that its price is equal to marginal utility, she could not be behaving optimally.Rather, they say, she would be better off quitting while she was ahead, or buying a quantity such that marginal utility is much greater than the price.What is wrong with this argument?

I think it's easier to answer this question if you have a graph to refer to.