American Board of Internal Medicine (ABIM) Certification Practice Exam

What significant characteristic of oncogenic osteomalacia is noted in patients?

1. High levels of 1,25-dihydroxy vitamin D

2. Normal phosphate levels

3. Kidney phosphate wasting

4. Overproduction of vitamin D

The correct answer is: Kidney phosphate wasting

Oncogenic osteomalacia is a rare paraneoplastic syndrome often associated with tumors, particularly mesenchymal tumors, that produce a phosphaturic substance. A significant characteristic of this condition is kidney phosphate wasting, which leads to hypophosphatemia. The phosphaturic factor inhibits renal tubular reabsorption of phosphate, resulting in its excretion in the urine. This phosphate wasting is coupled with low serum phosphate levels, which is a key feature of the disorder. Patients with oncogenic osteomalacia typically experience bone pain and other manifestations due to osteomalacia caused by the associated low phosphate levels. Additionally, while the condition can alter vitamin D metabolism, it is primarily characterized by the defective handling of phosphate in the kidneys rather than an overproduction of vitamin D or high levels of 1,25-dihydroxy vitamin D. Thus, kidney phosphate wasting is the hallmark of oncogenic osteomalacia, making it the correct characteristic to identify in this context.