Viral Myositis: This is the most likely diagnosis given the context of the question. Viral myositis, such as that caused by influenza, coxsackievirus, or other viral infections, can lead to elevated CPK (creatine phosphokinase) levels due to muscle inflammation and damage.

Muscle Strain or Trauma: Physical injury to the muscles can cause an elevation in CPK levels. This could be a consideration if the patient has a history of recent physical activity or trauma.
Toxic Myopathy: Certain medications and toxins can cause muscle damage, leading to elevated CPK levels. A thorough medication and exposure history is necessary to consider this diagnosis.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can cause muscle damage and elevated CPK levels, although this is less common.

Rhabdomyolysis: Although less likely, rhabdomyolysis is a serious condition that can cause extremely high CPK levels due to severe muscle breakdown. It can be life-threatening and requires immediate attention.
Muscular Dystrophy: Certain types of muscular dystrophy can present with elevated CPK levels. While less common, missing this diagnosis could have significant implications for long-term management and prognosis.
Inflammatory Myopathies (e.g., Dermatomyositis, Polymyositis): These autoimmune conditions can cause muscle inflammation and damage, leading to elevated CPK levels. They require specific treatment and can have significant morbidity if not addressed.

Glycogen Storage Diseases: Certain rare genetic disorders affecting glycogen metabolism can lead to muscle damage and elevated CPK levels.
Mitochondrial Myopathies: These are a group of rare disorders that affect the mitochondria, leading to muscle damage and potentially elevated CPK levels.
Statins-induced Myopathy: While not extremely rare, statin-induced myopathy is a consideration in patients taking statin medications, especially at high doses or in combination with other drugs that can increase the risk of myopathy.