Which condition indicates a horse exhibiting rigid posturing and third eyelids?

The correct choice indicates that tetanus is associated with rigid posturing and the presence of the third eyelids in horses. Tetanus, caused by the bacterium Clostridium tetani, leads to the production of a neurotoxin that affects the nervous system, resulting in muscle stiffness and spasms. The characteristic rigid posture is due to hypertonicity of the muscles, and the third eyelid protrudes due to increased muscle tension around the eyes.

In contrast, clostridial myositis typically manifests with swelling and potential necrosis of the muscle, often without the classic rigidity seen in tetanus. Hyperhidrosis refers to excessive sweating and does not typically cause rigid posturing or third eyelid protrusion. Botulism, while it can cause weakness and other neurological signs, usually leads to a flaccid paralysis rather than a rigid stance.

These distinctions clarify why tetanus is the condition that best matches the symptoms described.