Since we're all speculating with an
utter lack of actual information
I'll speculate that Donald Trump uses a prescription blood thinner or anti-platelet medication intended to prevent stroke. That causes minor bumps - hands, arms, legs, whatever - to become significant bruises. It would not limit such a person's activities, and the person on the medication could live for decades. Given Trump's age, I think my speculation is not crazy.
Speaking from experience and observation, when veins in the hand are used for IV whatever, I've
always seen the vein near the pinkie finger used, not the vein near the index finger.