This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Skin, Subject name: Medicine
A 27-year-old man develops a painless 1-cm sore on his penis. It appears ulcerated with a raised margin and minimal serous exudates. (See Figure below) Which of the following is the most appropriate next step in the diagnosis? 
Select and explain. 
A. biopsy, B. Gram stain, C. serology, D. ultrasound
C: serology
Answer: C: serology 
Explanation: This is likely a case of primary syphilis. Serology is the most appropriate test; it can remain negative for a period up to 1 month after the infection is contracted. The serologic test for syphilis usually is positive within 1 week after the chancre appears. With therapy, the chancre heals rapidly, but will heal in 4-6 weeks even without treatment. Genital chancres are usually painless, unless super-infected, but extragenital chancres (eg, fingers) can be quite painful. Biopsy is usually not necessary for diagnosis, and the spirochetes are seen with dark-field examination, not with Gram stain.