Sapundzhiev, Lyubomir, et al. “Problems Within the SpA Group – Diagnostic versus Classification Criteria; Differential Diagnosis Within the SpA Group; The Issue of Axial Psoriatic Arthritis”. Rheumatology (Bulgaria), vol. 34, no. 3, Sept. 2026, pp. 335-70, https://doi.org/10.35465/eb4ka042.