Seronegative Spondyloarthritis: Given the patient's symptoms of chronic severe polyarthralgia affecting both large and small joints, along with elevated inflammatory markers (CRP and ESR) and the absence of specific antibodies for lupus, Sjögrens, rheumatoid arthritis, or CREST syndrome, seronegative spondyloarthritis is a strong consideration. The ANA titer of 1:320 with an unrecognized pattern may not be specific enough to point towards a classic autoimmune disease but can be seen in various inflammatory conditions.

Osteoarthritis with Inflammatory Component: Although osteoarthritis is typically not associated with systemic inflammation, some patients can have an inflammatory component, especially if they are obese, which can exacerbate joint symptoms. The elevated CRP and ESR could support this, though the involvement of small joints is less typical.
Polymyalgia Rheumatica (PMR): PMR is characterized by stiffness and aching in the shoulders, neck, and hips, which could be considered under the broad term of polyarthralgia. The elevated ESR and CRP are consistent with PMR, but the age and the specific joint involvement might not perfectly align with typical PMR presentations.
Fibromyalgia: This condition is characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood issues. While fibromyalgia doesn't cause inflammation, the patient's symptoms of chronic severe polyarthralgia could be considered, especially in the context of obesity and the absence of clear inflammatory arthritis.

Lymphoma: Although less likely, lymphoma can present with systemic symptoms including joint pain, elevated inflammatory markers, and can sometimes have a positive ANA. It's crucial to consider this diagnosis due to its potential severity and the need for early intervention.
Multiple Myeloma: This condition can cause bone pain and elevated inflammatory markers. While less common, it's a diagnosis that should not be missed due to its significant implications.
Endocarditis: Infective endocarditis can present with joint pain, elevated inflammatory markers, and sometimes a positive ANA. It's a diagnosis that requires prompt recognition and treatment.

Relapsing Polychondritis: A rare autoimmune disorder characterized by recurrent episodes of cartilaginous inflammation, which could explain the polyarthralgia and elevated inflammatory markers.
Sjögren's Syndrome with Atypical Presentation: Although antibodies for Sjögren's were negative, some patients can have a negative serology but still have the disease, especially if they present atypically.
Mixed Connective Tissue Disease (MCTD): An autoimmune disease that combines features of lupus, scleroderma, and rheumatoid arthritis. The ANA positivity and polyarthralgia could be consistent with MCTD, though specific antibodies were negative.