1. Clinical data Patient, male, 18 years old. He came to our hospital with right knee pain. He complained of right knee pain for more than 6 years, mainly on the medial side of the right knee, and had been treated with “right knee arthritis” in several hospitals without any significant improvement. The patient recalled having slight pain in the left hip for more than 1 year 5 years ago, but did not pay attention to it because he was young. On examination: the pressure pain was obvious in the right knee, the left “4” test was weakly positive, there was no obvious pressure pain in the left groin, and slight pressure pain behind the left hip. HLA-B27 was positive, rheumatoid factor was negative, and anti-“O” was negative. The rest did not show any significant abnormalities. Ankylosing spondylitis (AS) is most commonly seen in adolescents, with a male to female ratio of about 14:1. The symptoms of the upstream type of AS begin with sacroiliac joint pain or lumbar pain, and in about 1/8 patients the symptoms begin at the knee. “In the early stage, the x-ray of sacroiliac joint shows that the joint margin is blurred and slightly dense, the joint space is widened, in the middle stage, the joint space is narrowed, the joint margin is interlaced with erosion and jagged, the bone dense zone on the side of the iliac bone is widened, the widest width is up to 3cm, in the late stage, the joint space disappears, the bone dense zone disappears, bone trabeculae pass through, and there is bony ankylosis; about 96% of HLA-B27 is positive in the serum of AS patients [1]. Most knee osteoarthritis in young people is due to definite trauma or rheumatoid in the knee joint. The reasons for the misdiagnosis of this case are analyzed by the author as follows: 1. The receiving physician did not follow up the medical history and physical examination in detail and mistakenly believed that knee pain with knee osteoarthritis was the most likely cause, ignoring the small probability that AS also presents with knee pain. 2. Some physicians may have believed that ipsilateral knee pain should occur when inflammatory lesions occur in the sacroiliac joint, so only the right side was examined during the physical examination “In fact, the inflammatory lesion occurred in the left sacroiliac joint of the patient, ignoring the fact that the compensatory effect caused excessive strain on the soft tissues around the right knee joint, resulting in right knee pain. 3. The doctor who sees the doctor takes a picture of the right knee because of the pain in the right knee, making the mistake of taking a picture of where the pain is.