Sherlock Season 4: 18 Easter Eggs & References From 'The Lying Detective'
14. Blessington The Poisoner
When at the children's ward at Smith's hospital, a very high Sherlock bungles the chance to tell the kids about his cases. One case that he spoils the end of involves Blessington the Poisoner.
The name Blessington is taken from Doyle's The Adventure of the Resident Patient. In that story, however, he is the victim and does not fall foul of poison.
Speaking of Smith's hospital, we see in one shot that it is called Saint Caedwalla's Hospital. Fittingly, Caedwalla is the patron saint of (repentent) serial killers. Yes, there is a saint for everything.