Are there dichotomous keys for identifying medical conditions?
For histopathology they use them all the time to describe cells and tissues, very much as they are used to describe biological entities.  Characteristics like size, cytoplasm to nucleus ratio, grouping, staining characteristics, immunohistochemistry and now molecular attributes are all used to categorize a cell of interest.  For cancer, the identity of the malignant cell is crucial to diagnosis and subsequent effective treatment - based on successful treatments used in the past for people with that type of cancer.

Here is a histopathology dichotomous key I found at Noelways.com