A medical tricorder is a specialist type of tricorder augmented for medical diagnosis purposes.
A medical tricorder’s sensors are designed for dedicated biological monitoring and analysis making the medical tricorder a key tool for federation physicians in the diagnosis of their patients.
One of the more obvious physical differences in a medical tricorder compared to a standard tricorder is that they often feature a small detachable scanning devise for more targeted scans whilst the tricorder continues its general analysis. 
In 2254 Spock borrowed Doctor Phillip Boyce's medical tricorder to confirm his diagnosis that the USS Enterprise's systems had been infected by an organic virus when the Ngultor attacked the ship. (EV comic: "Flesh of My Flesh")