When meticulously evaluating pelvic radiographs for templating a primary THA, the ‘teardrop’ is a critical landmark. What anatomical bony structure forms the distinct medial border of this radiographic teardrop?

Explanation:
The correct option is d. The radiographic U-shaped 'teardrop' is not a single anatomical structure, but rather a complex superimposition of osseous features on an AP pelvic radiograph. The lateral border of the teardrop represents the anteroinferior portion of the acetabular fossa. The crucial medial border represents the inner true pelvic cortex, which corresponds anatomically to the lateral wall of the obturator canal, formed primarily by the ischium. The inferior border is the acetabular notch. The teardrop is the primary radiographic landmark for determining the medial limit of acetabular reaming (preventing medial blowout) and assessing the vertical and horizontal center of rotation of the hip. Reference: Rubash HE et al. Radiographic assessment of the acetabulum in total hip arthroplasty. Clin Orthop Relat Res.