**Core Concept**
Multiple myeloma is a type of bone marrow cancer characterized by the proliferation of malignant plasma cells, leading to bone destruction and lytic lesions. The disease often results in the release of various cytokines and enzymes that promote osteoclast activity, bone resorption, and lytic bone lesions.

**Why the Correct Answer is Right**
The commonest site of lytic lesion in multiple myeloma is the skull, particularly the upper jaw (maxilla) and the upper portion of the occipital bone. This is due to the high concentration of plasma cells in the bone marrow of the skull and the resulting bone destruction caused by the release of osteoclast-activating factors. The skull's unique anatomy, with its thin cortical bone and high bone turnover rate, makes it more susceptible to lytic lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** The ribs, while a common site for metastatic disease, are not the most common site for lytic lesions in multiple myeloma. This is because the ribs have a relatively low concentration of bone marrow and are less affected by the disease.
**Option B:** The spine is a common site for lytic lesions in multiple myeloma, particularly in the thoracic and lumbar regions. However, it is not the most common site, which is the skull.
**Option C:** The pelvis is a common site for lytic lesions in various types of cancer, including multiple myeloma. However, it is not the most common site for lytic lesions in this disease.

**Clinical Pearl / High-Yield Fact**
The presence of lytic lesions in the skull, particularly in the upper jaw and occipital bone, is a classic radiographic finding in multiple myeloma. This finding, combined with the patient's clinical presentation and laboratory results, can help establish the diagnosis of multiple myeloma.

**Correct Answer:** A. Skull