An 18 year old is brought to the emergency room following an automobile accident. Serum enzyme studies demonstrate a thousand-fold elevation of creatine kinase (CK). Which of the following tissues is most likely to be the source of the enzyme?

Creatine kinase occurs in several different forms, or isoenzymes, in the body. Most tissues contain a mix of creatine kinase isoenzymes, but one species often predominates. The MB isoenzyme of creatine kinase is associated with hea damage; the MM isoenzyme is associated with muscle damage; and the BB isoenzyme is associated with brain damage. Surgery and trauma are commonly implicated as causes of sufficient skeletal muscle trauma to markedly elevated creatine kinase. If isoenzyme studies are done, the CK-MM (skeletal muscle isoenzyme) isoenzyme will be elevated much more markedly than the CK-MB isoenzyme. Bone is not a significant source of CK. Brain can occasionally be a source of creatine kinase (BB isoenzyme), but would not usually be the major source following an automobile accident. Hea damage as in myocardial infarction (unlikely at this age), can elevate creatine kinase, but would not usually be the major source in an automobile accident. If the dilemma arises as to whether an automobile accident was possibly secondary to a myocardial infarction, the ratio of CK-MB to total CK may be helpful in demonstrating a cardiac contribution (CK-MB is characteristic of hea muscle). Ref: Molina P.E. (2013). Chapter 3. Anterior Pituitary Gland. In P.E. Molina (Ed),Endocrine Physiology, 4e.