The Scuola Grande di San Marco is one of the confraternities of Venice--societies where laypeople…read more(men, to be precise) would gather to work toward charitable causes and goals in the past. Today it is part hospital (which, for obvious reasons, isn't open for tourist visits) and part medical history museum. The upstairs museum area was once home to some of Jacopo Tintoretto's most famous artworks, including The Miracle of the Slave, but those are now homed elsewhere. You can still find works by his son Domenico in the Scuola related to St. Mark and the "return" of his body to Venice.
For those interested in medical history, the display cases of old instruments, texts and documents can make for a fascinating visit. On a recent visit there was a wonderful temporary exhibit on Tintoretto and Art, Faith and Medicine during his time 500 years ago. The exterior Renaissance facade of the Scuola is quite beautiful as well. I only give it three stars, however, because if this is your first or only/short visit to Venice, I wouldn't put it high up on your "must see" list. It's a curious side-note to Venetian history but not one of the city's main attractions.