What is the most likely diagnosis for an 80-year-old female with progressive muscle weakness and an elevated creatine kinase level?

The most likely diagnosis for the scenario involving an 80-year-old female with progressive muscle weakness and an elevated creatine kinase level is statin-induced myopathy.

Statin medications, commonly prescribed for cholesterol management, can lead to muscle-related side effects, including muscle weakness and increased creatine kinase levels, especially in older patients or those with other risk factors. The age of the patient is particularly relevant, as older adults are at a higher risk for adverse effects from medications, including statins. Moreover, statin-induced myopathy typically presents with progressive muscle weakness, aligning with the symptomatology described in this case.

In contrast, inflammatory myopathy and muscular dystrophy generally present with different characteristics and may have additional clinical features, such as pain or specific muscle distribution weaknesses, making them less likely in this presentation. Hypothyroid myopathy often presents with a unique set of symptoms such as muscle stiffness or cramping and might not prominently feature elevated creatine kinase unless hypothyroidism is severe. Therefore, the combination of factors here—progressive weakness, advanced age, and elevated creatine kinase—strongly suggests statin-induced myopathy as the most plausible diagnosis.