Differential Diagnosis for Acute Onset of Muscle Pain in a 71-year-old Male Recently Prescribed Bactrim

• Single Most Likely Diagnosis
  • Drug-induced myopathy (e.g., due to Bactrim): This is a likely cause given the recent prescription of Bactrim, which can cause muscle pain as a side effect, especially in older adults.
• Other Likely Diagnoses
  • Polymyalgia rheumatica: This condition is common in older adults and presents with muscle pain and stiffness, particularly in the shoulders and hips.
  • Musculoskeletal strain: Given the patient's age, a recent strain or injury could easily occur and cause muscle pain.
  • Inflammatory myopathies (e.g., polymyositis, dermatomyositis): Although less common, these conditions can present with acute muscle pain and should be considered, especially if there are other symptoms like skin rash or systemic symptoms.
• Do Not Miss Diagnoses
  • Rhabdomyolysis: This is a life-threatening condition that can be caused by various factors, including certain medications like Bactrim. It presents with severe muscle pain, weakness, and dark urine.
  • Statin-induced myopathy: If the patient is also on statins, this could be a contributing factor, and it's crucial to recognize due to the potential for severe muscle damage.
  • Infections (e.g., pyomyositis, bacterial myositis): Although less likely, infections can cause acute muscle pain and are critical to diagnose promptly to initiate appropriate treatment.
• Rare Diagnoses
  • Mitochondrial myopathies: These are a group of rare genetic disorders that affect the mitochondria and can cause muscle pain, among other symptoms.
  • Glycogen storage diseases: Certain types of these diseases can present with muscle pain and weakness, although they are rare and typically diagnosed at a younger age.
  • Eosinophilia-myalgia syndrome: This is a rare condition associated with certain medications and can cause severe muscle pain, among other symptoms.