Differential Diagnosis

The patient's presentation of diffuse muscle pain, right hip and femur pain, positive ANA and PL7, and negative inflammatory markers suggests an autoimmune or rheumatologic condition. Here are the potential diagnoses, categorized for consideration:

• Single most likely diagnosis
  • Polymyositis (PM) or Dermatomyositis (DM): The presence of muscle pain, positive ANA, and specific anti-synthetase antibody (PL7) points towards an inflammatory myopathy, with PM or DM being strong contenders. The absence of skin manifestations would lean more towards PM.
• Other Likely diagnoses
  • Systemic Lupus Erythematosus (SLE): Although the patient's presentation is not classic for SLE, the positive ANA is a hallmark of the disease. The lack of other positive inflammatory markers or specific SLE criteria makes this less likely but still a consideration.
  • Mixed Connective Tissue Disease (MCTD): This condition overlaps with SLE, PM/DM, and other autoimmune diseases. The presence of anti-synthetase antibodies and ANA positivity, along with muscle pain, could suggest MCTD, especially if other criteria such as arthralgias, Raynaud's phenomenon, or esophageal dysmotility are present.
• Do Not Miss
  • Multiple Myeloma or Other Malignancies: Although less likely given the autoimmune antibody profile, it's crucial not to miss a potential malignancy, especially with bone pain (right hip and femur). Myeloma or metastatic disease could present with bone pain and elevated inflammatory markers, though the latter are negative in this case.
  • Osteonecrosis: Particularly of the femoral head, which could present with hip pain. This condition might not directly relate to the positive autoimmune markers but is a critical diagnosis to consider due to its potential for significant morbidity.
• Rare diagnoses
  • Eosinophilic Myopathy or Other Rare Myopathies: These conditions are less common and might present with muscle pain and specific antibody profiles. The presence of eosinophilia or other unique features on muscle biopsy could point towards these diagnoses.
  • Necrotizing Autoimmune Myopathy (NAM): A rare condition characterized by muscle weakness and necrosis without significant inflammation. It can be associated with specific autoantibodies and might be considered if other diagnoses are ruled out and muscle biopsy shows characteristic findings.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory findings, and potentially, additional diagnostic tests such as muscle biopsy or imaging studies.