This study is for patients with multiple myeloma (MM), a type of blood cancer, who need stem cell mobilization for a transplant. The goal is to compare two medications used before giving motixafortide, which helps collect stem cells. Motixafortide sometimes causes allergic reactions or side effects at the injection site. The two medications being compared are dexamethasone and methylprednisolone, both corticosteroids, which means they help reduce inflammation and allergic reactions. Dexamethasone is standard but can cause side effects like headaches or high blood pressure. The study will see if methylprednisolone is safer or more effective. Patients are divided into two groups to receive one of these medications before motixafortide.

• Study involves several visits over 4-8 days.
• Participants will undergo blood sample collection.
• Possible side effects include reactions from motixafortide and corticosteroids.