What is the correct combining form for the study of knowledge?

The combining form that represents the study of knowledge is derived from the Greek word "gnosis," which means knowledge. Therefore, "gnos/o" is used in medical terminology to refer to knowledge or understanding, especially in the context of diagnosis, where medical professionals apply their knowledge to identify a patient's condition.

In contrast, the other options pertain to different fields within medical terminology. "lip/o" is associated with fat, "kerat/o" relates to the cornea or hard tissue, and "myc/o" refers to fungi. Each of these prefixes has a distinct meaning that does not pertain to the concept of knowledge or its study. This makes "gnos/o" the only correct choice in this context, as it specifically addresses the notion of knowledge in medical terms.