What is the most likely diagnosis for a patient presenting with progressive muscle weakness and proximal limb weakness along with high serum creatine kinase?

The clinical presentation of progressive muscle weakness, particularly with an emphasis on proximal limb weakness, alongside elevated serum creatine kinase levels, strongly suggests a diagnosis of polymyositis. Polymyositis is an idiopathic inflammatory myopathy characterized by muscle inflammation and weakness. The proximal muscles of the shoulders and hips are often affected first, leading to difficulties with activities such as lifting the arms or climbing stairs.

The elevated serum creatine kinase is indicative of muscle damage, which is a common finding in patients with polymyositis as the damaged muscle fibers release this enzyme into the bloodstream. This, combined with the muscle weakness described, fits the profile for polymyositis more than the other conditions listed.

In the context of the other conditions: chronic inflammatory demyelinating polyneuropathy (CIDP) typically presents with distal weakness and sensory changes due to nerve demyelination rather than proximal muscle weakness and high creatine kinase. Inclusion body myositis can cause similar weakness but is usually characterized by more subtle features and tends to occur in older adults often with an insidious onset and variable response to immunosuppressive treatments. Myasthenia gravis typically causes fluctuating muscle weakness, particularly affecting ocular muscles, but does not usually present with elevated creat