Muscle injury or muscle disease: This is the most likely diagnosis because AST (aspartate aminotransferase) is found in both liver and muscle tissues. An elevation in AST with normal liver enzymes (ALT, total bilirubin, and alkaline phosphatase) suggests that the source of the elevation is not the liver but rather muscle injury or disease, such as muscle trauma, strenuous exercise, or muscular dystrophy.

Hemolysis: Although less common, hemolysis can cause an elevation in AST due to the release of AST from lysed red blood cells. This would be considered if there are other signs of hemolysis, such as anemia, elevated lactate dehydrogenase (LDH), or indirect bilirubin.
Myocardial infarction: AST can be elevated in myocardial infarction due to the release of AST from damaged heart muscle cells. This diagnosis would be considered in the context of chest pain, electrocardiogram (ECG) changes, or troponin elevation.

Wilson's disease: Although rare, Wilson's disease can cause an elevation in AST due to liver involvement. It's crucial not to miss this diagnosis because it can lead to severe liver damage if not treated promptly. Other signs might include low ceruloplasmin levels, Kayser-Fleischer rings, or neurological symptoms.
Alpha-1 antitrypsin deficiency: This genetic disorder can cause liver disease and might present with elevated AST. It's essential to consider this diagnosis, especially in younger patients or those with a family history of liver disease.

Mitochondrial myopathies: These are a group of rare diseases that affect the mitochondria, leading to muscle weakness and potentially elevated AST due to muscle damage.
Glycogen storage diseases: Certain types of glycogen storage diseases can affect both liver and muscle, leading to elevated AST. These are rare genetic disorders that typically present in childhood.

Clinical evaluation: A thorough clinical evaluation to assess for signs of muscle injury, liver disease, or other systemic conditions.
Laboratory tests: Additional laboratory tests such as creatine kinase (CK) to evaluate for muscle damage, hemoglobin, and hematocrit to assess for hemolysis, and specific tests for Wilson's disease or alpha-1 antitrypsin deficiency if suspected.
Imaging studies: Depending on the clinical context, imaging studies like ultrasound or MRI of the liver or muscles might be helpful.
Biopsy: In some cases, a liver or muscle biopsy might be necessary to establish a definitive diagnosis.