MUC5AC and Keratoconjunctivitis sicca: More broadly, DED can be categorised into four subtypes: (1) lipid-deficient dry eye, resulting from abnormalities in the tear film’s lipid layer; (2) aqueous-deficient dry eye, characterised by insufficient tear production, including cases of primary mucin deficiency; (3) dry eye induced by allergies or environmental irritants; and (4) dry eye associated with eyelid surface anomalies, including primary epitheliopathy and structural abnormalities of the eyelids.