Alex Weed, who plays leukemia patient Chris Santos, is married to Fiona Gubelmann (Dr. Morgan Reznick) in real life.
ARDS is short for Acute Respiratory Distress Syndrome is a condition in which fluid collects in the lungs' air sacs, depriving organs of oxygen.
According to this episode's a lot, "Black Santa" was diagnosed with a "sigmoid volvulus," a bowel obstruction. Overall, sigmoid (sigma is the Greek letter equivalent to the letter "S," refers to bowel section resemblance to the letter "S," hence the name sigmoid) is the next to last segment of the colon before undigested matter leaves the intestines.
The diabetic patient, played by Peter Ciuffa, calls Shaun "Doogie", This is clearly a reference to Doogie Howser, M.D. (1989) about a genius who becomes a doctor at 16, who struggles to be accepted by patients. This is something Shaun faces himself due to looking young.