Metastatic Cancer: This is often considered the most likely diagnosis due to the combination of bone pain, fatigue, and loss of appetite, which are common symptoms in advanced cancer stages, particularly in cancers that frequently metastasize to bone like breast, prostate, and lung cancer.

Multiple Myeloma: A plasma cell disorder that can cause bone pain due to lytic lesions, fatigue from anemia, and loss of appetite. It's a common consideration in patients with these symptoms, especially in older adults.
Osteoporosis: While more commonly associated with fractures, severe osteoporosis can cause bone pain. However, the combination with significant fatigue and loss of appetite might suggest a more systemic issue, but it remains a consideration, especially in post-menopausal women or individuals with risk factors.
Chronic Kidney Disease (CKD): CKD can lead to bone pain (due to secondary hyperparathyroidism and renal osteodystrophy), fatigue, and loss of appetite. The disease's progression can explain these symptoms, especially if there's a known history of kidney issues.

Multiple Myeloma (again, due to its potential severity and the importance of early diagnosis): Despite being listed as another likely diagnosis, its severity and the importance of not missing it warrant its inclusion here as well.
Bone Metastases from an Unknown Primary: Identifying the primary source of cancer is crucial for treatment planning. Missing this diagnosis could lead to delayed appropriate therapy.
Vitamin D Deficiency: While not immediately life-threatening, severe deficiency can lead to significant morbidity, including bone pain and fatigue. It's easily treatable and thus important not to miss.

Paget's Disease of Bone: A chronic condition that can result in enlarged and misshapen bones due to abnormal bone destruction and regrowth. It can cause bone pain and, less commonly, systemic symptoms like fatigue and loss of appetite, especially if there's a significant skeletal involvement.
Osteogenesis Imperfecta (in adults): Though more commonly diagnosed in childhood, adults can present with symptoms due to osteogenesis imperfecta, including bone pain and fractures. It's rare and might not immediately come to mind but is a consideration in the differential diagnosis of bone pain.
Gastric Cancer: Though less common than other cancers, gastric cancer can metastasize to the bone and cause these symptoms. It's a rare consideration but one that should not be overlooked, especially in populations with a high incidence of gastric cancer.