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: Microbiology
A 23year old male had unprotected sexual intercourse with a commercial sex worker. Two weeks later, he developed a painless, indurated ulcer on the glans that exuded clear serum on pressure. Inguinal lymph nodes in both groins were enlarged and not tender. The most appropriate diagnostic test is - 
Select and explain. 
A. Gram's stain of ulcer discharge, B. Dark field microscopy of ulcer discharge, C. Giemsa stain of lymph node aspirate, D. ELISA for HIV infection
B: Dark field microscopy of ulcer discharge
Answer: B: Dark field microscopy of ulcer discharge 
Explanation: None