Elevated CPK in rheumatology most directly indicates involvement of which tissue?

• A. Liver
• B. Skeletal muscle
• C. Kidney
• D. Heart

Answer: (B)

When tissues are damaged, their intracellular enzymes leak out into the blood. Creatine phosphokinase (CPK) is most abundant in muscle tissues, with the highest proportion in skeletal muscle (the CK-MM isoenzyme). In rheumatologic conditions that attack skeletal muscle—such as inflammatory myopathies like polymyositis or dermatomyositis—the damaged muscle fibers release large amounts of CPK, so total CPK levels rise directly because of skeletal muscle injury.

In contrast, CK is also present in the heart (CK-MB) and brain (CK-BB), but those sources are more specific to cardiac or brain injury. In a rheumatology context, the pattern and clinical setting point to skeletal muscle as the primary source of the elevated CPK. Liver involvement is not a primary source of CK elevations, since CK is not their hallmark enzyme.