Hypothesis # 2 Fetlock joint capsule rupture. Mare suspected of stepping on foal's leg. Fibrous integrity of capsular tissue compromised by excessive shearing force. Capsular fluid is lost into surrounding tissues / structures, as the capsular space is now in communication with the extracapsular space. Sensory nerves within the capsular tissue initiate pain response. Disturbed blood vessels within capsular tissue initiates chemotactic response for neutrophils / basophils, as numerous inflammatory mediators are released and synthesized by endothelial cells and surrounding mast cells. Increased pressure results from fluid extravasation, pain response to damaged capsular tissue, and inflammatory mediator release into circulation are all sequelae this insult.