A 28-year-old man with a history of intravenous drug use presents with a 1-week history of fever, malaise, and new-onset breathlessness. On examination, he has a temperature of 38.7°C, a pansystolic murmur loudest at the lower left sternal edge which increases on inspiration, and tender erythematous nodules on his fingertips. What is the most likely valvular lesion involved?