A 4-month-old male infant presented with diffuse waxy, sclerotic skin lesions on throughout his body, sparing the face, nipples, and genital area. These lesions had first appeared when the boy was 2 months of age.

Histopathologic findings of the sclerotic skin lesions showed wide, fibrous intersecting bands in the subcutaneous fat tissue. In addition, sparse infiltration of lymphocytes and histiocytes were observed in the fat lobules and septa.

Which other investigations should be advised for this patient?

What should be the differential diagnosis?