I don't know of one's existence, I think your gonna have to rely on clinical knowledge. Metro for anaerobes below the belt, Macrolides for community acquired pneumonia, things like that.
One thing I have noticed is that everyone in hospitals uses commercial names for drugs, while in school and for boards they only use the generic names. Metronidazole is called Flagyl, and Ceftriaxone is called Rocephin, stuff like that. Having and medscape app or something similar is a must.