A 52-year-old man from Delhi presents with a painless swelling in the right groin that has been present for 6 months. The swelling appears when he stands or strains and disappears on lying down. On examination, the swelling is above and medial to the pubic tubercle, and it does not extend into the scrotum. A cough impulse is present. The examiner notes that the swelling lies within the inguinal canal. Which of the following structures forms the medial boundary of the inguinal canal?