Base texture of an asset

I want to get the base texture image of an object. If the material referenced by this object has shaders of UsdReviewSurface, I can know which shader has the base texture from the diffuseColor attribute. However, if the shaders are SourceAsset and reference an mdl file, it is much more complicated. Based on that, my question is how can I find the shader having the base texture image if the shaders inside the referenced material are SourceAsset and reference an mdl file. I am using pixar api to do most of the coding.

Thank you