Tyree has been in a state of incessant worry for almost half a year now. He has trouble sleeping and breathing. Dr.Simons, a psychiatrist, asks Tyree detailed questions about his symptoms and medical history. Tyree tells Dr.Simons that he cannot stop worrying and is constantly nervous. Tyree also complains of muscular tension and stomach problems. After conducting a quick examination of Tyree's symptoms, Dr.Simons discovers that Tyree's incessant worry does not have a particular cause, and it has taken a physical toll on Tyree's health. Which of the following disorders is Dr.Simons most likely to diagnose Tyree with?
a. Schizophrenia
b. Bipolar disorder
c. Obsessive-compulsive disorder
d. Generalized anxiety disorder