43. A 30-year-old male athlete comes to the clinic complaini…

43. A 30-year-old male athlete comes to the clinic complaining of sharp pain in his elbow after repeatedly throwing a baseball during practice.  The pain is worsened when he tries to extend his arm. The physician has determined the pain is related to inflammation in the structure that attaches the muscle to the bone.  Which of the following structures is involved in this injury?