WNK2 and psoriatic arthritis: From 2019 to 2021, the field turned to research on (1) psoriatic arthritis; (2) iron; (3) Qing Dai, baicalin, β-sitosterol, polysaccharide, polyphenol, and berberine; (4) long noncoding RNA; (5) dectin-1, caspase-1, cytotoxic T-lymphocyte antigen 4, adenosine monophosphate-activated protein kinase, mitogen-activated protein kinase, signal transducer and activator of transcription 3 (STAT3), phosphatase and tensin homolog, and bcl-2-associated X protein; (6) Janus kinase-2; (7) IL-1β, IL-17, and IL-13.