Let's identify the discrepancy: Even though life spans have increased and overall health has improved--due to advances in medical research and technology, which have allowed the medical profession to diagnose and treat a wide variety of illnesses and injuries--over the past few decades, there has been a steady and significant increase in the rate of serious infections.
So the correct answer should explain why despite the fact that life spans have increased and overall health has improved because of advances in medical research and technology improving the ability the medical profession to diagnose and treat a wide variety of illnesses an injuries, over the past few decades, there has been a steady and significant increase in the rate of serious infections.
Answer choice (D) states: "As a population increases in size, there is a directly proportional increase in the number of serious infections."
This explains why over the past few decades there has been a steady and significant increase in the rate of serious infections, but it does not explain why this is happening when advances in medical research and technology have improved the ability of the medical profession to diagnose and treat a wide variety of illnesses and injuries.
All we know is that life spans are increasing. We cannot use this to infer that the population is increasing in size, merely that the population is living longer.
Answer choice (E) states: "Modern treatments for many otherwise fatal illnesses increase the patient's susceptibility to infection."
This answer choice explains why during the time that advances in medical research and technology has bettered the ability of the medical profession to diagnose and treat a wide variety of illnesses and injuries, a steady and significant increase in the rate of serious infections has also occurred.
We know that advances in medical research have allowed for new treatments of otherwise fatal illnesses, and these new treatments increase one's chances of contracting an infection. Thus, this answer choice helps resolve the discrepancy.
Hope that clears things up! Please let us know if you have any other questions.
