American Board of Psychiatry and Neurology (ABPN) Practice Exam 2025 - Free ABPN Practice Questions and Study Guide

The most likely diagnosis for the patient described is inclusion body myositis. This condition typically presents in older adults, often beyond the age of 50, and is characterized by progressive weakness in the proximal and distal muscles. In this patient’s case, the reported weakness is more pronounced in the distal muscles, which is a key feature that aligns with inclusion body myositis.

Inclusion body myositis can also lead to mildly elevated serum creatine kinase (CK) levels, although the elevation is usually less pronounced compared to other inflammatory myopathies like polymyositis. Most importantly, the pattern of muscle weakness and the patient’s age make inclusion body myositis the most fitting diagnosis. Diagnostic confirmation often comes from a muscle biopsy that reveals specific findings such as rimmed vacuoles, which are characteristic of this condition.

Other diagnoses can present with muscle weakness and elevated CK levels, but they typically have distinguishing features or demographics that help differentiate them from inclusion body myositis. For instance, myasthenia gravis primarily results in fluctuating muscle weakness and is not characterized by the same degree of muscle atrophy or creatine kinase elevation. Dermatomyositis and polymyositis also show more proximal weakness and are more common