This is a difficult question, but the correct answer choice is clearly E.

Consider the following analogy, perhaps not the best one but I hope it will be good enough to make the point.

A dealer sells second-hand cars. Last year 75% of the cars didn't present any problems after the sale, whereas the other 25% presented several problems a few months after the sale. Clearly in that 25% percent of the cars sold, the dealer was more interested in making profit that in the security of the people who bought the cars.

Which of the following, if true, most seriously undermines the argument?

-- the dealer didn't know whether the cars had any problems prior to the sale.
-- the dealer equally informed all the customers about the risks of buying a second-hand car.

In this example, I know is not exactly the same, the correct answer choice is clearly the first one. The dealer here could know that the cars were in bad condition, and still convinced the people to buy the cars.

What makes option D incorrect?

If you read closely the stimulus is says

Thus it appears that for one in four such patients, the doctors who advised them to undergo this surgery, with its attendant risks and expense.

IMO the stimulus already acknowledges for the information given, so the doctors could haven given the same information but recommended patients undergo to the coronary bypass surgery even if the doctors knew that the patients would not benefit.