qualifying chronic disability
(2) For purposes of this subsection, the term “qualifying chronic disability” means a chronic disability resulting from any of the following (or any combination of any of the following): (A) An undiagnosed illness. (B) A medically unexplained chronic multisymptom illness (such as chronic fatigue syndrome, fibromyalgia, and irritable bowel syndrome) that is defined by a cluster of signs or symptoms. (C) Any diagnosed illness that the Secretary determines in regulations prescribed under subsection (c) warrants a presumption of service-connection.