Perianal Pain Associated With Crohn Colitis: What Next?

What would your diagnosis be in this young man and how would that inform your next step?

A 28-year-old man with Crohn colitis on maintenance therapy with infliximab 5 mg/kg every 8 weeks presents with perianal pain. On examination, he has a 1-cm tender lesion with fluctuance near the anus. 

Which of the following should be the next step in management?