This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Shoulder and arm injuries, Subject name: Orthopaedics
[{'content': 'Most common muscle affected by fibrosis is \nSelect and explain. \nA. Serratus anterior, B. Trapezius, C. Sternocleidomastoid, D. Tendocalcaneous', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Fibrosis of sternocleisomastoid occurs in Less than 0.4 percnt of all newborns but it&; is the most common cause of congenital toicollis The sterno-mastoid muscle on one side of the neck is fibrosed and fails to elongate as the child grows, and thus results in a progressive deformity. The cause of fibrosis is not known, but it is possibly a result of ischaemic necrosis of the sterno-mastoid muscle at bih. Evidence in our of this theory is the presence of a lump in the sterno-mastoid muscle in the first few weeks of life, probably a swollen ischaemic muscle. Ref: Maheshwari and Mhaskar 9th ed pg 321.', 'role': 'assistant'}]