A 44 year old woman shows low serum calcium, low phosphate, and raised alkaline phosphatase. What is the most likely diagnosis?

The clinical presentation of low serum calcium, low phosphate, and elevated alkaline phosphatase is indicative of osteomalacia. This condition is characterized by the inadequate mineralization of bone, often due to vitamin D deficiency, which leads to the observed low levels of calcium and phosphate.

In osteomalacia, the elegant balance between bone formation and mineralization is disrupted; although osteoblasts remain active and produce bone matrix, the lack of sufficient calcium and phosphate prevents proper mineralization, resulting in weak bones. The elevated alkaline phosphatase level reflects increased osteoblastic activity attempting to mineralize the bone, which is a hallmark of osteomalacia.

In contrast, Paget's disease typically presents with even higher alkaline phosphatase due to excessive bone remodeling but not necessarily with low calcium and phosphate. Osteoporosis is primarily characterized by a decrease in bone density without the alteration in calcium, phosphate, or alkaline phosphatase levels observed in this case. Multiple myeloma may present with different electrolyte abnormalities and commonly features osteolytic lesions rather than variations in bone mineralization parameters like those seen in osteomalacia.

Thus, the combination of low serum calcium, low phosphate, and increased alkaline phosphatase strongly supports the diagnosis of osteomalacia