In a 54‑year‑old man from Germany with a >10‑year history of progressive, steroid‑responsive inflammatory myopathy (near‑complete remission on prednisone 60 mg, relapse at 20 mg), mild creatine kinase elevation, elevated myoglobin, liver enzyme elevation, PET‑CT showing FDG uptake in neck, shoulder, forearm and adductor muscles, Raynaud phenomenon, aortic ectasia with valve calcification, coronary calcification, exertional dyspnea, bilateral leg edema (lymphatic insufficiency), obesity (BMI ≈ 38), obstructive sleep apnea, hypertension, and lack of response to methotrexate and sulfasalazine, what are the most likely differential diagnoses, the optimal diagnostic work‑up, and evidence‑based steroid‑sparing treatment recommendations?

{
  "question": "Act as a medical AI assistant for clinicians, specializing in chronic inflammatory/muscular diseases. Analyze this case and provide evidence-based diagnosis/treatment recommendations. PATIENT [AGE]/M, [COUNTRY], >10y progressive disease, steroid-responsive Main goal: Clarify inflammatory condition with muscle involvement SYMPTOMS Systemic: Severe fatigue/weakness, exercise intolerance, post-exertional crash, fluctuating Muscular: Progressive weakness, stair tremor, \"lead legs\" post-exertion; PET-CT: FDG-uptake (neck/shoulder/forearm/adductors) Vascular/Rheum: Raynaud, aortic ectasia/valve calcification, coronary calc., exertional dyspnea Edema/Skin: Bilateral leg edema (lymphatic insuff.), stasis dermatitis, ?psoriasis, photosens.-like FINDINGS Labs: CK [VALUE] U/l (↑), Myoglobin [VALUE] ng/ml (↑), CRP low/nl, Liver enz. ↑ (GOT/GPT/GGT), Thyroid nl Imaging: PET-CT → myositis-like, no LVV/PMR; Vascular US: no thrombos./obstr., lymphatic edema THERAPY Prednisolone [DOSE]mg: Near-complete remission (symptoms/edema); Rel