Final answer:
E. Heart is the most medial structure among the given options, situated closer to the center of the chest, whereas other options pertain to limbs, which are located more laterally in relation to the midline of the body.
Step-by-step explanation:
The most medial structure among the options provided is E. Heart.
An anatomical explanation involves how the body is positioned and how the limbs develop. Initially, in embryonic development, the future palm of the hand and the bottom of the foot face medially, meaning towards the midline of the body.
The upper limb then laterally rotates by 90 degrees, positioning the palm anteriorly and the thumb laterally, while the lower limb rotates medially, aligning the big toe with the medial side. However, the heart, which often is thought to be on the left side, is actually situated more centrally within the chest, making it the most medial compared to the third finger, fifth toe, hand, and ears.