A 60 year old lady with a history of HTN and suffering from RA since the last 10 years now presents with hot, swollen and tender knee joint. What investigation would you do for her?

The key is B. C&S of joint aspirate. [RA itself is a risk factor for joint infection and there are possibilities to increase the chance of septic arthritis with the use of probable steroid which lowers immunity and further facilitates infection of joints for which C&S of joint aspirate should be done].