Erysipeloid (Erysipelothrix rhusiopathiae)

A 38-year-old fishmonger presents with a 5-day history of a well-demarcated, violaceous plaque on the dorsum of his right hand and index finger following a puncture wound from a fish bone 2 days prior. The lesion has central clearing, advances slowly at the periphery, and stops at the wrist. He reports mild burning pain and stiffness but denies fever, lymphangitis, or regional lymphadenopathy. The structure marked **B** in the diagram represents the characteristic presentation of erysipeloid. Which of the following is the MOST APPROPRIATE initial antimicrobial therapy for this condition?