A 52-year-old man from Delhi presents with a 3-week history of progressive weakness in his right leg, particularly affecting hip extension and knee flexion. On examination, he has marked weakness of the gluteus maximus and biceps femoris muscles. Sensation is diminished over the posterior and lateral aspects of the leg and dorsum of the foot. He denies bowel or bladder involvement. An MRI of the pelvis reveals a 4 cm mass compressing the sciatic nerve at the level of the greater sciatic foramen. Which of the following is the most likely site of nerve compression?