A Patient with Chronic Low Back Pain Due to Instability

A patient with chronic low back pain due to instability presents to physical therapy with complaint of unilateral knee pain without history of injury to the knee.Tested in sitting the patient has full active range of motion.Tested in standing the patient has limited range of motion and pain response.The most reasonable explanation is

A) malingering response in standing.
B) primary knee pathology present only in standing.
C) fatigue in standing increases pain.
D) abnormal compensatory movement pattern in standing.