**Core Concept**
The patient's symptoms suggest a primary bone disorder characterized by lytic lesions, recurrent infections, and an increased risk of fractures, pointing towards a diagnosis of Multiple Myeloma (MM). MM is a cancer of plasma cells that accumulate in the bone marrow, leading to bone destruction and anemia.

**Why the Correct Answer is Right**
The investigation that will best help in establishing a diagnosis of Multiple Myeloma is a **Bone Marrow Biopsy**. This procedure involves removing a sample of bone marrow from the hipbone using a needle. The biopsy sample is then examined under a microscope to look for abnormal plasma cells. The biopsy can confirm the presence of plasma cell neoplasia and help identify the specific type of myeloma. Additionally, it can provide information on the extent of bone marrow involvement and the presence of any other related conditions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Serum Protein Electrophoresis (SPEP) is a useful diagnostic tool for detecting monoclonal proteins, but it is not the most definitive investigation for establishing a diagnosis of Multiple Myeloma.
* **Option B:** Magnetic Resonance Imaging (MRI) can help identify lytic lesions in the skull and other bones, but it does not provide a definitive diagnosis of MM.
* **Option D:** Computed Tomography (CT) scans can also identify lytic lesions, but like MRI, they do not provide a definitive diagnosis of MM.

**Clinical Pearl / High-Yield Fact**
In patients with suspected Multiple Myeloma, a high level of **Monoclonal Gammopathy of Undetermined Significance (MGUS)** can be a precursor to MM, and patients with MGUS should be closely monitored for the development of MM.

**Correct Answer:** B. Magnetic Resonance Imaging (MRI) can help identify lytic lesions in the skull and other bones, but it does not provide a definitive diagnosis of MM.