Which medical term is associated with the combining form "phac/o"?

The combining form "phac/o" is derived from the Greek word "phakos," which refers specifically to a lens. In medical terminology, it pertains to the eye, particularly when discussing conditions related to the lens, such as cataracts. The lens is crucial for focusing light onto the retina, and any problems associated with it can significantly affect vision.

The other options represent different anatomical structures or concepts. "Vein" relates to the vascular system, "phalanges" refer to the bones of the fingers and toes, and "sound" is related to auditory sensations. None of these terms shares a connection with the concept of the lens or eye health, affirming that "phac/o" is accurately associated with the lens in medical terms.