OCD lesions arise when rapid skeletal growth disrupts normal endochondral ossification, leading to cartilage flaps or loose bone pieces in the joint. In foals and young horses, this process is most active during the first year of life, and many lesions become detectable around the middle of that period. Seven months sits squarely in this active growth window, making changes radiographically apparent as the articular cartilage thickens and the subchondral bone begins to show irregularities. That’s why seven months is the most commonly cited time for development or diagnosis. The earlier ages are usually too young for reliable detection, while nine months is less typical, though not impossible.

In OCD, different parts of the joint are subjected to different kinds of mechanical stress, and this shapes how lesions develop. Flap lesions form where shear forces across the cartilage-subchondral interface are high, causing cartilage to fissure and eventually detach as a loose piece. Subchondral bone cysts arise under areas of maximal compressive loading, where repeated compression leads to microfractures in the subchondral bone and intrusion of fluid, forming a cyst beneath the cartilage. So the best statement reflects both patterns: flaps in high shear regions, and cysts in areas subjected to maximal compressive loads. The other ideas don’t fit because they tie flaps or cysts to the opposite loading conditions.

When a dog cannot open its mouth, the first priority is to look for mechanical problems in the jaw and skull that would physically prevent opening. Imaging the skull with radiographs lets you visualize the mandible, temporomandibular joints, maxilla, and teeth to detect fractures, dislocations, severe dental disease, or osteomyelitis. These findings are common and treatable causes of trismus, and they guide the next steps in management. If the skull radiographs show no structural issue, you then investigate non-mechanical causes such as inflammatory or autoimmune muscle diseases, which would be pursued with specific tests like antibody titers. So skull radiographs are the best next test because they directly assess the most likely surgical or traumatic causes and quickly direct subsequent diagnostic decisions.

HYPP from SCN4A mutations alters the skeletal muscle voltage-gated sodium channel (Nav1.4). The SCN4A gene encodes the alpha subunit that gates sodium influx necessary for action potential initiation. Mutations often cause a gain-of-function effect—abnormal persistent sodium current or delayed inactivation—leading to abnormal depolarization of muscle fibers. When serum potassium rises, the depolarized state reduces excitability and produces episodic weakness or paralysis. Since SCN4A encodes a sodium channel, the primary channel affected is the sodium channel; potassium, calcium, and chloride channels are not the direct targets of this mutation.