A 48-year-old obese male presents with a 6-week history of severe inferomedial heel pain that is worst when he takes his first few steps in the morning, but improves with continued walking. He works as a security guard and stands for 8–10 hours daily. On examination, there is tenderness over the medial calcaneal tubercle, and pain is elicited on dorsiflexion of the toes. The structure marked **D** in the diagram is the primary static support of the medial longitudinal arch. Which of the following is the MOST LIKELY diagnosis and the MOST APPROPRIATE first-line management?