**Core Concept**
The patient's presentation of lytic lesions in the lumbar back and pelvis, along with elevated serum levels of alkaline phosphatase, suggests a possible bone metastasis. The presence of keratin-positive cells on aspiration biopsy further points towards a metastatic carcinoma.

**Why the Correct Answer is Right**
The patient's symptoms and laboratory findings, including elevated alkaline phosphatase, are consistent with bone metastasis. The presence of keratin-positive cells on aspiration biopsy is a hallmark of epithelial origin, which is typical of metastatic carcinomas. The normal serum calcium and protein levels rule out primary hyperparathyroidism or multiple myeloma, respectively. The lack of other specific markers or features makes metastatic carcinoma the most likely diagnosis.

**Why Each Wrong Option is Incorrect**

**Option A:** Chondrosarcoma is a primary malignant tumor of cartilage, which is less likely to present with lytic lesions and elevated alkaline phosphatase. Chondrosarcomas typically have a different radiologic appearance and may show specific markers such as cartilage matrix production.

**Option C:** Osteochondroma is a benign tumor of cartilage and bone, which is not typically associated with lytic lesions or elevated alkaline phosphatase. It may present as a solitary lesion or a hereditary syndrome, but it is not a likely diagnosis in this case.

**Option D:** Osteosarcoma is a primary malignant tumor of bone, which can present with lytic lesions and elevated alkaline phosphatase. However, osteosarcomas typically have a different radiologic appearance and may show specific markers such as osteoid production, making metastatic carcinoma a more likely diagnosis.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with lytic lesions and elevated alkaline phosphatase, consider the possibility of metastatic carcinoma, especially if there is a history of a known primary malignancy. The presence of keratin-positive cells on aspiration biopsy can be a key diagnostic clue.

**✓ Correct Answer: B. Metastatic carcinoma**