MSK & Connective Tissue Practice Questions

A 32-year-old man presents with a 9-month history of chronic low back pain that is worse in the morning, improves with activity, and awakens him from sleep. He also reports stiffness and limited range of motion in his lumbar spine. Physical examination reveals tenderness over the sacroiliac joints and decreased spinal mobility. X-rays of the pelvis show bilateral sacroiliitis. Which of the following human leukocyte antigen (HLA) alleles is most strongly associated with this condition?

A 16-year-old boy presents with progressive pain and swelling in his left distal femur for the past 3 months. He denies any history of trauma. Radiographs of the knee show a destructive lesion in the metaphysis of the distal femur with a 'sunburst' pattern and Codman's triangle. A biopsy is performed. Which of the following cell types is the malignant component of this tumor?

A 5-year-old boy is brought to the pediatrician by his parents due to progressive muscle weakness. They report that he has difficulty climbing stairs, frequently falls, and uses his hands to push on his thighs to stand up from a squatting position (Gowers' sign). Physical examination reveals pseudohypertrophy of the calves. Serum creatine kinase levels are markedly elevated. Which of the following genetic defects is most likely responsible for this patient's condition?