Posterior tibialis tendon dysfunction (PTTD) is a common "runner's nemesis" causing inner ankle pain, arch flattening, and weakness, often resulting from overpronation, overuse, or improper footwear. It presents as pain along the tendon that worsens with running, sometimes leading to a visible loss of the foot arch. Early, proactive treatment with rest, strengthening, and orthotics is critical to prevent long-term, irreversible arch collapse. Pain, swelling, and tenderness, specifically behind the medial malleolus (inner ankle bone), which may increase during or after running.