Section: USMLE

20) A 75-year-old man presents to his physician with complaints of nocturia, urinary urgency, and a feeling that he could not completely empty his bladder. A digital rectal exam reveals a firm, enlarged prostate. A bone scan is ordered and shows positivity in multiple vertebral bodies. Elevation of which of the following substances would be most strongly associated with the development of bone lesions?

1. Prostatic acid phosphatase
2. Prostate-specific antigen
3. Serum alkaline phosphatase
4. Tartrate-resistant acid phosphatase
5. Urinary hydroxyproline

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 and prostatic-specific antigen 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 and urinary hydroxyproline 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.