A 24-year-old woman presents with anterior knee pain and a sensation of the knee "giving way" during squatting. MRI of the knee is performed. The structure marked **B** in the diagram shows smooth, uniform signal intensity with intact surface morphology. However, on a different axial slice at the patellofemoral joint, focal softening and fibrillation of the articular surface is noted. Which of the following best describes the imaging finding and its most likely clinical diagnosis?