Background: This proof-of-concept study examined the association between CT-derived sarcopenia indices and achievement of Patient Acceptable Symptom State (PASS) in a metabolically homogeneous subgroup of patients with psoriatic arthritis (PsA) receiving biologic therapy. Methods: We retrospectively analyzed 26 PsA patients without diabetes, hypertension, or thyroid disorders who were treated with TNF or IL-17 inhibitors and underwent opportunistic CT imaging at the L2 vertebral level. Body composition parameters included total abdominal muscle area (TAMA), subcutaneous adipose tissue (SAT), and sarcopenia index (SI = TAMA/height²). PASS achievement at 12 weeks was assessed retrospectively. Receiver operating characteristic (ROC) analysis was performed. Results: Fourteen patients achieved PASS. Compared with non-achievers, these patients had significantly higher TAMA, SAT, and SI values (all p < 0.05). ROC analysis identified an SI threshold of 4657.5 cm²/m² (sensitivity 78.6%, specificity 83.3%, p = 0.003). Conclusions: In this metabolically homogeneous PsA subgroup, higher CT-derived muscle mass measures were associated with PASS achievement at 12 weeks. These findings are hypothesis-generating and require validation in larger prospective cohorts.