USMLE Answer 20
20. The correct answer is C. The patient has prostate cancer causing osteoblastic bone lesions. Osteoblastic cells respond to metastatic prostate carcinoma by forming bone (osteoid) and secreting alkaline phosphatase, which is thought to either initiate or facilitate mineralization.
Prostatic acid phosphatase (choice A) and prostatic-specific antigen (choice B) are not correct because they do not answer the question being asked. The question asks for bone metabolites related to the patients skeletal metastasis. These two markers are synthesized by the tumor and would most likely be elevated in this case; however, they are elevated because of the prostatic cancer, independent from the bony metastasis.
Tartrate-resistant acid phosphatase (choice D) and urinary hydroxyproline (choice E) are metabolic markers of osteoclastic (not osteoblastic) cell activity. Lytic tumor metastasis (lung, kidney, gastrointestinal tract, melanoma) would be associated with increased levels of these markers. Tartrate-resistant acid phosphatase is secreted by the osteoclast during bone resorption. Hydroxyproline is associated with collagen breakdown, and increased levels are excreted in the urine.