A child presented in the OPD with multiple permeating lesions involving all the bones of the body which of the following is the most probable diagnosis :
Neuroblastoma
Metastasis from osteosarcoma
Histiocytosis X
Metastasis from Wilm's tumour
Osteolytic lesions in a child involving all the bones of the body points towards the diagnosis of Histiocytosis X.
The bony lesions in Histiocytosis x can present as a single lesion or as a wide spread disease involving virtually any bone of the body.
The lesions are osteolytic and the common sites of involvement are --> Skull, Vertebrae, Ribs & Clavicle.

​About other choices :

Metastasis from Wilm's tumour  -   It can be easily be excluded as the metastatic from Wilms' tumour does not involve bones.
Neuroblastoma  -  Though it can involve any bone of the body, but simultaneously involvement of all the bones of the body without any common manifestation is very unlikely to be neuroblastoma.
Metastasis from Osteosarcoma - Metastasis from osteosarcoma commonly goes to lung not bones.