Answer:
insulin
Explanation:
diabetes prevents the body from producing enough insulin to regulate blood sugar.
Thousands of Americans are on the Waiting list for an Organ Transplant.
All of these but the nosebleeds seem correct, but something much stronger like cocaine can cause nosebleeds.
But if you want just ONE answer 'impaired memory and learning' seems like the best one to pick because that is one of the most noticeable signs of marijuana use.
This website talks about the symptoms of use, I trust its accuracy:
https://www.webmd.com/mental-health/addiction/marijuana-use-and-its-effects#1