This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
A lady presents with right knee swelling. Aspiration was done in which CPPD crystals were obtained. Next best investigation is - 
Select and explain. 
A. ANA, B. RF, C. CPK, D. TSH
D: TSH
Answer: D: TSH 
Explanation: 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.