A 70-year-old women admitted with worsening anemia and pathological fracture of the humerus, had an ESR of 120 mm in 1st hour. Her peripheral smear showed increased rouleaux formation. X ray of skull showed multiple punched out osteolytic lesions. a. What is the most probable diagnosis? b. Write briefly on the etiopathogenesis and describe the bone marrow changes in this disease.