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 plot, "Black Santa" was diagnosed with a "sigmoid volvulus," a bowel obstruction. Overall, sigmoid (sigma is the Greek letter equivalent to the letter "S," which refers to the 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 refers to Doogie Howser, M.D. (1989) about a genius who becomes a doctor at 16 and struggles to be accepted by patients--something Shaun faces due to looking young.