A] The change in dosage increased the amount of the drug required, making the treatment significantly more expensive.
Does not explain why skin worsens, only that the treatment is more expensive.
B] The amount of medication used does not significantly affect the ease with which patients can follow their treatment regimens.
Does not explain why skin worsens, only that the medication does not affect the ability for patients to follow instructions.
C] Most of the patients participating in Skinworth’s study have severe problems with acne and are especially vulnerable to sudden flare-ups.
Does not explain, this is just a statement of fact.
D] When too much of the drug is used, it irritates the skin, causing bumps and splotching.
Agreed -- this is correct. You'd think that if something helps you, the more you use it, the more it'd help you. However, this is clearly not the case, as expressed in the argument, and D explains why: if you use too much of the drug, it will irritate the skin and worsen its condition.
E] Some of the patients participating resigned from Skinworth’s study because they were unsatisfied with the results of the treatment.
Does not explain anything.
(Really, the patients' first mistake was taking directions from a company called "Skinworth"... ).