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: Skin
A 23-year-old male had unprotected sexual intercourse with a commercial sex worker. Two weeks later, he developed a painless, indurated ulcer on the glans which 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. Darkfield microscopy of ulcer discharge, C. Giemsa stain of lymph node aspirate, D. ELISA for HIV infection
B: Darkfield microscopy of ulcer discharge
Answer: B: Darkfield microscopy of ulcer discharge 
Explanation: B i.e. Dark field microscopy of ulcer discharge