Final answer:
The deltoid ligament is the most likely to be compromised in PTT dysfunction due to its strength and positioning. It is often involved in severe eversion sprains or injuries, contrasting with more common inversion sprains affecting other ligaments.
Step-by-step explanation:
The most likely ligament to be compromised with posterior tibial tendon (PTT) dysfunction is the deltoid ligament due to its anatomical positioning and the nature of the force involved in most ankle injuries. In the context of PTT dysfunction, it is important to differentiate between the common inversion sprains which often injure ligaments such as the anterior talofibular ligament, and eversion sprains or injuries that can affect the deltoid ligament. Eversion sprains are less common but can be quite severe, potentially involving a Pott's fracture due to forceful eversion of the foot with subsequent displacement of the talus and fracture of the distal fibula. This displacement can be a result of the strong deltoid ligament shearing off the medial malleolus rather than tearing, which indicates just how strong this ligament is.