American Board of Internal Medicine (ABIM) Certification Practice Exam

What urine phosphate excretion level is indicative of renal phosphate wasting when evaluating for conditions like oncogenic osteomalacia?

1. Less than 50 mg/d

2. Greater than 100 mg/d

3. Equal to 80 mg/d

4. Greater than 500 mg/d

The correct answer is: Greater than 100 mg/d

In the evaluation of renal phosphate wasting, especially in conditions such as oncogenic osteomalacia, the level of phosphate excretion in the urine is critical. A urine phosphate excretion level greater than 100 mg/d is indicative of renal phosphate wasting. This elevation suggests that the kidneys are not reabsorbing phosphate effectively, leading to excessive phosphate loss in the urine. In conditions like oncogenic osteomalacia, where there is typically a low serum phosphate due to increased renal loss, this level of phosphate excretion serves as an important diagnostic marker. A urine phosphate excretion above this threshold points to a renal handling issue rather than dietary factors or absorption problems from the gastrointestinal tract. Therefore, this differentiation is essential for accurate diagnosis and subsequent management of the underlying condition.