Most Likely Diagnosis: Prostatic Cancer with Bone Metastases
The most likely diagnosis is B. Prostatic cancer, specifically metastatic disease with bone involvement, given the markedly elevated PSA (84 ng/mL), significantly elevated alkaline phosphatase (410 IU/L), back pain indicating vertebral metastases, and obstructive urinary symptoms. 1
Diagnostic Reasoning
Laboratory Findings Point Definitively to Metastatic Prostate Cancer
- PSA of 84 ng/mL carries a >50% probability of prostate cancer and far exceeds the threshold of 10 ng/mL that indicates high cancer probability 1
- The alkaline phosphatase elevation to 410 IU/L (normal 39-117 IU/L) strongly suggests bone metastases, as this enzyme is released from osteoblastic activity in metastatic bone lesions 1
- The combination of obstructive urinary symptoms, back pain, and these laboratory findings creates a highly specific pattern for metastatic prostate cancer 1
Clinical Presentation Confirms Metastatic Disease
- Back pain in the context of prostate cancer typically indicates vertebral metastases 1
- The urinary obstruction results from local tumor growth causing bladder outlet obstruction 1
- Normal albumin and gamma-glutamyl transferase exclude hepatic metastases as the source of alkaline phosphatase elevation 1
Why Other Diagnoses Are Excluded
A. Prostatitis - Ruled Out
- Alkaline phosphatase would remain normal in prostatitis 1
- PSA elevations in prostatitis are typically modest (rarely >20 ng/mL) and acute in onset 2, 3
C. Urinary Bladder Cancer - Ruled Out
- Bladder cancer does not produce PSA elevations 2
- Bladder cancer does not cause elevated alkaline phosphatase unless there are bone metastases, which would be extremely unusual presentation 1
D. Benign Prostatic Hyperplasia - Ruled Out
- BPH does not cause elevated alkaline phosphatase 1
- BPH does not typically cause back pain 1
- While BPH can elevate PSA, levels are typically <10-20 ng/mL; a PSA of 84 ng/mL is incompatible with BPH alone 2, 3
Immediate Next Steps
Urgent Imaging Required
- Do not delay imaging for potential spinal cord compression - back pain with neurologic symptoms requires urgent MRI to exclude epidural metastases requiring emergency intervention 1
- Perform focused neurologic examination immediately to assess for cord compression signs 4
Mandatory Staging Studies
- Bone scintigraphy is mandatory given PSA >10 ng/mL and clinical suspicion of bone metastases 1
- Tissue diagnosis via transrectal ultrasound-guided biopsy should be obtained to confirm histology and grade 1
Critical Pitfall to Avoid
The most dangerous error would be delaying spinal imaging if any neurologic symptoms are present (leg weakness, saddle anesthesia, bowel/bladder dysfunction beyond simple obstruction), as epidural spinal cord compression from vertebral metastases requires emergency neurosurgical decompression within hours to prevent permanent paralysis 1