Fact : The recently injured athlete's drug test was positive for marijuana
Athlete's reasoning : spending some time at a party where others were smoking
Doctor's conclusion : contact of that sort could not result in a positive indicator.

Thus taking all these together the reason has to be something which has been administered to him because of his injury

(A) Taking large amounts of ibuprofen can result in false positives for marijuana. - Correct as gives alternate reason for his positive result

(B) Secondhand smoke exposes innocent bystanders to a variety of toxic chemicals. - Incorrect - toxic chemical ---- not marijuana

(C) Excessive exercise has been found to mask certain effects of marijuana use. - Irrelevant

(D) The athlete had tested positive for amphetamines three years earlier. - incorrect --- the question pertains to present testing

(E) The team doctor sends all test samples out to an independent laboratory. -- Irrelevant as veracity of tests are not being questioned

IMO A

The recently injured athlete's drug test was positive for marijuana. Although the athlete insisted that he had not used the drug, he did admit to spending some time at a party where others were smoking. The doctor, however, insisted that contact of that sort could not result in a positive indicator.

Which of the following, if true, would contribute most to an explanation of the results of the drug test?

(A) Taking large amounts of inuprofen can result in false positives for marijuana. Correct

because he was injured and may be took large amounts of inuprofen for pain release

(B) Secondhand smoke exposes innocent bystanders to a variety of toxic chemicals. Irrelevant

(C) Excessive exercise has been found to mask certain effects of marijuana use. Incorrect

if excessive exercise mask certain effects of marijuana, than how he tests positive

(D) The athlete had tested positive for amphetamines three years earlier. Incorrect

that is not our concern

(E) The team doctor sends all test samples out to an independent laboratory. Incorrect

it supports doctors statements

The argument doesn't talk about ibuprofen though. Argument should mention that ibuprofen is consumed in case of injuries. It is not a general knowledge by any means that GMAT would allow imo.

OFFICIAL EXPLANATION

(A) The athlete claims he did not use marijuana, and he could not have failed the drug test due to casual contact with the smoke. The explanation for the failed drug test must come from another source, and only choice (A) would provide justification for the result.

You do not need to know about ibuprofen. The argument asks us to determine whether an external source could have been responsible for the effects presented in the athlete.

Hovkial :-

The recently injured athlete's drug test was positive for marijuana. Although the athlete insisted that he had not used the drug, he did admit to spending some time at a party where others were smoking. The doctor, however, insisted that contact of that sort could not result in a positive indicator.

Taking large amounts of ibuprofen can result in false positives for marijuana. -> So? Did injured athlete take ibuprofen? If yes, then you can resolve the paradox. If no, then you still don't know whether something else was the cause for detection.

Basically you are assuming that the injured athlete consumed "large amounts of ibuprofen" because of which the test resulted in false positive and thus explains the paradox.

Please take the time to properly read the question.

Nothing is being assumed here. We are asked to determine whether an external cause could have been responsible for the concerned effect.

The question clearly states: "Which of the following, if true,...." The question asks you to assume the truth of a statement in the option.

I would urge you to do the same.

I never disagreed that "taking large amounts of ibuprofen will lead to false positives". The question stem doesn't say which of the following if true of an action performed by injured athlete. The final question that I ask myself is whether injured athlete took ibuprofen. What leads you to think that athlete actually took ibuprofen. Ask yourself.