<prompt> A lady presents with right knee swelling. Aspiration was done in which CPPD crystals were obtained. Next best investigation is -ANARFCPKTSH  <response>: Finding CPPD crystal in aspirated synovial fluid is diagnostic of CPPD arthropathy.CPPD arthropathy in some patients may be associated with :- Primary hyperparathyroidism, Hypothyroidism, Hemochromatosis, Hypomagnesemia, Hypophosphatasia.ANA antinuclear antibody is for SLE, RF rheumatoid factor is for rheumatoid arthritis and CPK creatinine phosphokinase is raised in dermatomyositis polymyositis.So, among the given option only TSH is the relevant investigation in context of CPPD arthropathy as it may be associated with hypothyroidism. 