A new weight-loss drug causes the loss of body fat in all patients who

For an inference question, try to stick as close to the given premises as you can-- any jump in logic must be fully justified. With that in mind, attack the choices:

Choice (A) uses one of those wishy washy phrases that can be hard to argue with: "not necessarily." Maybe people who take the drug will decrease their percentage of body fat, maybe not. Another phrase that should set off mental alarm bells here is "overall percentage"--I'm with Gryphon here. We've all of a sudden shifted from a discussion of absolute quantities (mass) in the argument to relative quantities (percentage). Look at this concrete example:

Bob weighs 200 pounds, 20 lbs of which is body fat. Bob's mass is therefore 10% (20/200) body fat.

He takes the drug and loses 1 pound of body fat, AND
9 pounds of lean muscle mass. He has lost a total of 10 pounds, so weighs 190 pounds, 19 pounds of which are body fat. He is still 10% (19/190) body fat. This is NOT a decrease in overall percentage of body fat.

BUT what about sridar's suggestion that the word "moderate" implies that the person loses more fat than lean body mass? In real life, this might be a perfectly reasonable assumption to make-- on the GMAT, however, AVOID making assumptions unless you absolutely have to. Even if you weren't convinced, leave choice A for now and look at the others.

Choice (B) also uses "not necessarily"--but here you can disprove it clearly. If the body's mass is composed entirely of body fat and lean body mass, and both decrease, then the person's weight WILL necessarily decrease. Eliminate.

Nothing is mentioned in the argument about the chemical composition of the drug, or any kind of targeting. Just because the drug causes loss of body fat doesn't mean it targets fat cells. Maybe it targets something else and the effect on fat cells is a side effect. (Look at the runaway success of Latisse, which was a glaucoma medication that happened to grow eyelashes, and now is a blockbuster vanity drug.) Choice (C) is out.

I couldn't put it any more succinctly than Gryphon did regarding choices (D) an (E), so won't try here. They're out too, for the reasons listed in the above post.

Despite sridar's very reasonable reservations about the word "moderate"-- the other choices have large fatal errors, and what "moderate" means is never laid out in the argument. A is the last choice left standing.