Dark field microscopy used in ?

Ans. is 'b' i.e., Syphilis Dark field microscopy is used for spirochetes (for example T pallidum which causes syphilis). Dark ground/ Dark field microscope Very slender organisms such as spirochetes and filamentous organs such as flagella are not visible under ordinary illumination. The contrast in dark field microscopy gives an illusion of increased resolution so that these structures can be seen under the dark field mcroscope. In dark field microscopy reflected light is used instead of the transmitted light used in the ordinary microscope.