Most Likely Diagnosis: Metastatic Prostate Cancer
The most likely diagnosis is prostate cancer (Option B), specifically metastatic disease with bone involvement. This patient's markedly elevated PSA of 84 ng/mL, significantly elevated alkaline phosphatase of 410 IU/L, back pain, and urinary obstruction symptoms form a classic presentation of advanced prostate cancer with skeletal metastases 1.
Diagnostic Reasoning
PSA Interpretation
- A PSA level >10 ng/mL carries a >50% probability of prostate cancer 1
- At PSA ≥50 ng/mL, the positive predictive value for prostate cancer reaches 98.5% 2
- This patient's PSA of 84 ng/mL makes prostate cancer virtually certain 2
Alkaline Phosphatase Elevation
- The markedly elevated alkaline phosphatase (410 IU/L, normal 39-117) strongly suggests bone metastases 1
- Guidelines recommend bone scintigraphy when PSA is >10 mg/L or when bone metastases are clinically suspected 1
- Back pain combined with elevated alkaline phosphatase is a classic presentation of skeletal metastatic disease 1
Clinical Presentation Analysis
- Back pain in the context of prostate cancer typically indicates vertebral metastases 1
- Urinary obstruction symptoms reflect local tumor burden causing bladder outlet obstruction 3
- The combination of obstructive urinary symptoms, back pain, and these laboratory findings creates a highly specific pattern for metastatic prostate cancer 1
Why Other Diagnoses Are Unlikely
Prostatitis (Option A)
- Prostatitis can elevate PSA, but rarely to this degree 4
- Alkaline phosphatase would remain normal in prostatitis 1
- Back pain is not a typical feature of prostatitis 1
Bladder Cancer (Option C)
- PSA is produced exclusively by prostatic epithelial cells, not bladder tissue 4
- Bladder cancer does not cause PSA elevation 4
- This diagnosis is incompatible with the laboratory findings 4
Benign Prostatic Hyperplasia (Option D)
- While approximately 25% of BPH patients have PSA >4 ng/mL, levels of 84 ng/mL are exceptionally rare in pure BPH 1, 4
- BPH does not cause elevated alkaline phosphatase 1
- BPH does not typically cause back pain 1
- The constellation of findings makes BPH an inadequate explanation 1
Immediate Next Steps
Staging Workup Required
- Bone scintigraphy is mandatory given PSA >10 mg/L and clinical suspicion of bone metastases 1
- Tissue diagnosis via transrectal ultrasound-guided biopsy (minimum 8 cores) should be obtained 1
- However, in carefully selected patients with PSA >50 ng/mL and clear metastatic disease, androgen ablative therapy may be initiated without biopsy if the patient has severe comorbidities or spinal cord compression 2
Critical Pitfall to Avoid
- 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