USMLE Step 2 CK: Haematology/Oncology—Tumour Lysis Syndrome, Hypercalcemia of Malignancy, Spinal Cord Compression

What This Is and Why It Matters for USMLE

Tumour Lysis Syndrome (TLS), Hypercalcemia of Malignancy (HCM), and Spinal Cord Compression (SCC) are high-yield topics for Step 1, Step 2 CK, and Step 3. They frequently appear in basic science, clinical, and ethics/management contexts on the USMLE. Understanding these conditions is crucial for managing patients with cancer.

High-Yield Facts (What You Must Memorize)

Tumour Lysis Syndrome (TLS)

• Pathophysiology: Rapid cell lysis releases intracellular contents (ATP, potassium, phosphate, and nucleic acids) into the bloodstream.
• Classic presentation: Hyperkalemia, hyperphosphatemia, hypocalcemia, hyperuricemia, and acute kidney injury.
• Diagnostic approach:
  • Labs: Elevated creatinine, potassium, phosphate, and uric acid.
  • Imaging: Not typically used.
• First-line treatment and management:
  • Allopurinol to prevent uric acid production.
  • Rasburicase to decrease uric acid levels.
  • Fluid resuscitation to prevent hyperkalemia.
  • Potassium and phosphate binding agents.
• Red flags, complications, and follow-up:
  • Monitor renal function and electrolytes.
  • Correct electrolyte imbalances.

Hypercalcemia of Malignancy (HCM)

• Pathophysiology: Parathyroid hormone-related protein (PTHrP) or parathyroid hormone (PTH) production by cancer cells.
• Classic presentation: Nausea, vomiting, abdominal pain, constipation, and altered mental status.
• Diagnostic approach:
  • Labs: Elevated calcium levels, normal or low PTH.
  • Imaging: Bone survey to identify metastases.
• First-line treatment and management:
  • Hydration with IV fluids.
  • Bisphosphonates (e.g., zoledronic acid) to decrease bone resorption.
  • Calcitonin to decrease calcium levels.
• Red flags, complications, and follow-up:
  • Monitor renal function and calcium levels.
  • Correct electrolyte imbalances.

Spinal Cord Compression (SCC)

• Pathophysiology: Tumor growth or metastasis compressing the spinal cord.
• Classic presentation: Back pain, radiculopathy, and neurological deficits (e.g., weakness, numbness, bladder/bowel dysfunction).
• Diagnostic approach:
  • Labs: Elevated erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP).
  • Imaging: MRI or CT scan to identify the level and extent of compression.
• First-line treatment and management:
  • Steroids to reduce inflammation.
  • Radiation therapy to reduce tumor size.
  • Surgery to decompress the spinal cord.
• Red flags, complications, and follow-up:
  • Monitor neurological function.
  • Correct any electrolyte imbalances.

Clinical Pearls & Buzzwords

• Tumor lysis syndrome: ATP release, hyperkalemia, hyperphosphatemia, hypocalcemia.
• Hypercalcemia of malignancy: PTHrP, PTH, bone metastases.
• Spinal cord compression: Cord compression, radiculopathy, neurological deficits.

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation.
2. Generate a differential (most likely and must-not-miss).
3. Order appropriate initial tests.
4. Interpret results.
5. Initiate treatment and monitoring.

Missing a life-threatening complication (e.g., TLS, HCM, SCC) can lead to poor patient outcomes.

Common Mistakes & Exam Traps

• The mistake: Failing to recognize the classic presentation of TLS (e.g., hyperkalemia, hyperphosphatemia).
• Why it happens: Misunderstanding the pathophysiology of TLS.
• How to avoid it: Review the classic presentation and pathophysiology of TLS.
• Exam board insight: The examiners will test your ability to recognize the classic presentation and pathophysiology of TLS.
• The mistake: Failing to order appropriate initial tests for HCM (e.g., calcium levels, PTH).
• Why it happens: Misunderstanding the diagnostic approach for HCM.
• How to avoid it: Review the diagnostic approach for HCM.
• Exam board insight: The examiners will test your ability to order appropriate initial tests for HCM.
• The mistake: Failing to initiate timely treatment for SCC (e.g., steroids, radiation therapy).
• Why it happens: Misunderstanding the first-line treatment and management of SCC.
• How to avoid it: Review the first-line treatment and management of SCC.
• Exam board insight: The examiners will test your ability to initiate timely treatment for SCC.

How It’s Tested on USMLE

• Step 1: Basic science vignette (e.g., molecular mechanism, pathology slide, pharmacology).
• Step 2 CK: Clinical vignette (e.g., "A 45-year-old with chest pain...").
• Step 3: Similar to Step 2 CK, plus prognosis, risk factors, and occasionally CCS management.

CCS (Step 3) Relevance (If Applicable)

• Initial orders: Order basic labs (e.g., electrolytes, renal function), imaging (e.g., MRI or CT scan), and initiate treatment (e.g., steroids, radiation therapy).
• Monitoring and follow-up: Monitor neurological function, correct any electrolyte imbalances, and adjust treatment as needed.
• Common mistakes: Failing to order indicated tests, delaying treatment, and not monitoring neurological function.

Practice Questions (3-5 single-best-answer)

Question 1: A 35-year-old with acute leukemia develops TLS. Which of the following is the most appropriate initial treatment? A) Allopurinol B) Rasburicase C) Fluid resuscitation D) Potassium binding agents

Answer: C) Fluid resuscitation Explanation: Fluid resuscitation is the most appropriate initial treatment for TLS to prevent hyperkalemia.

Question 2: A 60-year-old with breast cancer develops HCM. Which of the following is the most appropriate initial treatment? A) Hydration with IV fluids B) Bisphosphonates C) Calcitonin D) Steroids

Answer: A) Hydration with IV fluids Explanation: Hydration with IV fluids is the most appropriate initial treatment for HCM to decrease calcium levels.

Question 3: A 50-year-old with lung cancer develops SCC. Which of the following is the most appropriate initial treatment? A) Steroids B) Radiation therapy C) Surgery D) Chemotherapy

Answer: A) Steroids Explanation: Steroids are the most appropriate initial treatment for SCC to reduce inflammation.

Quick Reference Card (60-Second Summary)

• TLS: ATP release, hyperkalemia, hyperphosphatemia, hypocalcemia.
• HCM: PTHrP, PTH, bone metastases.
• SCC: Cord compression, radiculopathy, neurological deficits.
• First-line treatment for TLS: Allopurinol, Rasburicase, Fluid resuscitation.
• First-line treatment for HCM: Hydration with IV fluids, Bisphosphonates.
• First-line treatment for SCC: Steroids, Radiation therapy.

If You Get Stuck on Test Day

• Eliminate obviously wrong answers: Use the process of elimination to narrow down the options.
• Use the "next best step" hierarchy: Start with the least invasive option and work your way up.
• For Step 3 CCS: Order basic labs, imaging, and initiate treatment as needed.

Related USMLE Topics

• Hypercalcemia of malignancy connects to sickle cell disease through hyperphosphatemia.
• Spinal cord compression connects to multiple sclerosis through neurological deficits.
• Tumor lysis syndrome connects to acute kidney injury through hyperkalemia.