Differential Diagnosis
- Single most likely diagnosis
- Rhabdomyolysis: This is the most likely diagnosis given the patient's symptoms of painful myalgia, difficulty walking, and dark brown urine after intense physical activity (running a 10-kilometer race). The laboratory findings of significantly elevated creatine kinase (CK) levels (57,000 U/L), hyperphosphatemia, hypocalcemia, and the presence of pigmented granular casts in the urine further support this diagnosis. Rhabdomyolysis is a condition characterized by the breakdown of muscle tissue, leading to the release of muscle fiber contents into the blood, which can cause kidney damage.
- Other Likely diagnoses
- Immune-mediated necrotizing myopathy: Although less likely than rhabdomyolysis given the context, this condition could also present with elevated CK levels and muscle weakness. However, it typically does not follow intense physical activity and may have a more gradual onset.
- Inflammatory myopathy: This could be considered if the patient had a history of autoimmune diseases or if the symptoms did not directly correlate with the physical activity. However, the acute onset after a race makes this less likely.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- March hemoglobinuria: Although rare and less likely given the clinical context (e.g., the presence of myalgia and extremely high CK levels), march hemoglobinuria could cause dark urine. It is typically seen in military recruits or individuals who march long distances and is due to the breakdown of red blood cells. Missing this diagnosis would not be as critical as missing rhabdomyolysis, given the treatment and prognosis differences.
- Rare diagnoses
- Polymyositis or dermatomyositis: These are inflammatory myopathies that could present with muscle weakness but are less likely given the acute onset after physical activity and the lack of skin manifestations (in the case of dermatomyositis).
- Toxin or drug-induced myopathy: While the patient is on atorvastatin, which can cause myopathy, the acute presentation after intense exercise and the degree of CK elevation point more towards rhabdomyolysis as the primary diagnosis. However, the contribution of atorvastatin to the development of myopathy should not be entirely ruled out.