uk-dgm MCQ (SBA): Inclusion Body Myositis Diagnosis

A 72-year-old man with a CFS of 3 presents to the GP with a 6-month history of bilateral lower limb weakness, particularly when climbing stairs. He also reports difficulty swallowing large tablets. He has no sensory symptoms. Examination reveals proximal muscle weakness (MRC grade 4/5 bilateral hip flexors, 4/5 bilateral deltoids), normal reflexes, and no fasciculations. CK is mildly elevated at 450 U/L. What is the MOST likely diagnosis?