challenge, 1. a method of testing the sensitivity of an individual to a hormone, allergen, or other substance by administering a sample. A small amount may be injected to determine whether the immune system will react by producing appropriate antibodies. 2. a term used to describe the rapid or concentrated infusion of a substance such as potassium or magnesium in the face of a life-threatening deficiency or a rapid infusion of IV fluid to differentiate between fluid deficit or renal failure as the cause of severely decreased urine output. 3. a physical or mental/emotional obstacle.
