Final answer:
In a modified radical mastectomy, the breast, axillary nodes, and superior apical nodes are removed but the muscles are preserved.
Step-by-step explanation:
The nurse should explain to the patient that in a modified radical mastectomy, option B is correct. In this procedure, the breast, axillary nodes, and superior apical nodes are removed, but the muscles are preserved. The major and minor pectoral muscles are not preserved, as mentioned in option C. Option A is incorrect because in a modified radical mastectomy, both the breast and the lymph nodes are removed. Option D is incorrect because there is no mention of splitting the sternum or dissecting lymph nodes from the mediastinum in this specific procedure.