This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Haemophilus, Yersinia, Spirochaetes, Ricketssia, Chlamydia, Mycoplasma and Miscellaneous Bacteria), Subject name: Microbiology
Dark ground microscopy is used for: 
Select and explain. 
A. TPI, B. Kahn's test, C. FTA-ABS, D. VDRL
A: TPI
Answer: A: TPI 
Explanation: In TPI test , the serum of patient with complement and T.pallidum maintained in a complex medium anaerobically . If the antibodies are present , the treponemes are rendered non motile, when examined under dark ground illumination Kahn test and VDRL test are non specific antigen antibody test ,cardiolipin antigen is used. FTA-ABS is specific antigen antibody test