What does the abbreviation "AS" not refer to?

The abbreviation "AS" commonly refers to "left ear" in the context of audiology and healthcare. This terminology is derived from the Latin "auris sinistra," which means the left ear. Other abbreviations in this setting include "AD" for the right ear ("auris dextra") and "AU" for both ears ("auris utraque").

Given this context, "Auditory System," which is represented by the abbreviation "AS" in the question, does not match the common usage associated with ear designations. Instead, the auditory system refers broadly to the anatomical structures and pathways involved in hearing, but it does not have an established abbreviation of that nature in clinical settings.

Thus, recognizing that the other terms specifically denote ear locations and not the auditory system clarifies why "D" is the option that does not apply.