Gross Anatomy USMLE Step 1 Practice Question

A 28-year-old man presents to the emergency department with a 5-day history of symmetric, ascending lower extremity weakness that began in his feet and has progressively worsened. He reports no recent travel, tick exposure, or ill contacts. On examination, temperature is 37.1°C, blood pressure 124/78 mmHg, heart rate 84/min, and respiratory rate 16/min. He has diminished deep tendon reflexes throughout both lower extremities and normal upper extremity strength. Gait is unsteady due to leg weakness. A lumbar puncture is planned to evaluate for Guillain-Barré syndrome. As the needle is advanced in the midline between the L3 and L4 spinous processes, the physician notes a firm resistance just before the needle enters the epidural space and breaks through to obtain clear cerebrospinal fluid. Which of the following anatomical structures is responsible for the resistance felt immediately before entering the epidural space?