MSK Individual Readiness Assurance Test (iRAT) Practice Test

Ionophore toxicity primarily injures muscle tissue because these compounds disrupt ion transport across cell membranes, causing calcium overload in muscle cells. In horses, cardiac and skeletal muscles are especially vulnerable due to their high energy demands and tightly regulated calcium handling. The influx of calcium triggers mitochondrial dysfunction, energy failure, and activation of catabolic enzymes, leading to myofiber necrosis. Clinically, this presents as weakness, exercise intolerance, tachycardia, and can progress to heart failure or sudden death. Blood tests often show marked elevation of muscle enzymes like CK and AST, and myoglobinuria may occur from rhabdomyolysis. The liver and kidneys are not the primary targets of ionophore damage, and neural tissue is not typically affected, so the hallmark pattern is necrosis of cardiac and skeletal muscle.