Everything You Need to Know About Multiple myeloma for Step 1

Multiple myeloma is one of those Step 1 “classic” diagnoses that shows up everywhere: renal failure with bone pain, recurrent infections, weird protein electrophoresis patterns, and that one buzzword everyone remembers—Bence Jones proteins. But the real test-day advantage comes from understanding why it causes each finding (and how to distinguish it from look-alikes like MGUS, Waldenström macroglobulinemia, and metastatic bone disease).

Big Picture (What it is + why it matters)

Multiple myeloma (MM) is a plasma cell malignancy characterized by clonal proliferation of plasma cells in the bone marrow that produce a monoclonal immunoglobulin (most commonly IgG or IgA) and/or free light chains.

High-yield themes for USMLE:

• Bone destruction via osteoclast activation → lytic lesions, fractures, hypercalcemia
• Renal injury from light chains and hypercalcemia → AKI, CKD
• Infection risk from impaired normal antibody production → encapsulated organisms
• Characteristic labs: M spike, rouleaux formation, high total protein, low albumin-to-globulin ratio

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First Aid cross-reference: Hematology/Oncology → Plasma cell disorders (Multiple myeloma section), monoclonal gammopathies, SPEP/UPEP, rouleaux formation, lytic bone lesions.

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Definition & Diagnostic Core

Core definition

A malignant plasma cell neoplasm with:

• Clonal plasma cells in bone marrow (often $\ge 10\%$), plus
• End-organ damage attributable to the plasma cell disorder

The testable diagnostic framework: CRAB

End-organ damage classically tested as CRAB:

• C: Hypercalcemia
• R: Renal failure
• A: Anemia
• B: Bone lesions (lytic)

Modern criteria also include biomarkers (often beyond Step 1), but CRAB remains the most testable.

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Pathophysiology (Mechanisms you can use to reason through questions)

1) Monoclonal plasma cells take over the marrow → anemia + infection risk

• Malignant plasma cells crowd out normal hematopoiesis → normocytic anemia
• They also suppress normal immunoglobulin production → hypogammaglobulinemia of functional antibodies
  • Paradox: you may see high total immunoglobulin, but it’s mostly monoclonal and nonfunctional

Clinical consequence: recurrent infections, classically encapsulated bacteria (e.g., S. pneumoniae).

2) Osteoclast activation → lytic lesions, fractures, hypercalcemia

Myeloma cells secrete cytokines that increase osteoclast activity (and inhibit osteoblasts), including:

• IL-1
• IL-6 (also a growth factor for plasma cells)
• RANKL upregulation (via marrow stromal interactions)

Result: punched-out lytic lesions, vertebral compression fractures, and hypercalcemia.

3) Light chains damage kidneys → “myeloma kidney”

Monoclonal free light chains (often Bence Jones proteins) are filtered and can:

• Form casts → cast nephropathy
• Deposit as amyloid (AL amyloidosis—see below)

Extra renal pearls:

• Light chains are not always detected well on serum protein electrophoresis—they often show up on UPEP or serum free light chain assay.

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Clinical Presentation (What Step stems look like)

Classic symptoms & signs

• Bone pain (especially back/ribs), pathologic fractures
• Fatigue (anemia)
• Recurrent infections
• Constipation, confusion, polyuria (hypercalcemia)
• Foamy urine or renal symptoms (proteinuria, renal failure)

High-yield associations

• Rouleaux formation on peripheral smear (RBC stacking)
  • Caused by increased serum proteins decreasing zeta potential
• Increased ESR (common clue when viscosity/protein is high)
• Weight loss and constitutional symptoms can appear but aren’t the hallmark like in many lymphomas

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Diagnosis (Step-style workup and what each test “means”)

Key labs and findings (high yield)

• CBC: normocytic anemia
• CMP: elevated creatinine; elevated calcium
• Total protein: often elevated; low A/G ratio
• SPEP/UPEP: monoclonal protein (M protein)
• Bone imaging: lytic lesions

Serum/Urine Protein Electrophoresis (SPEP/UPEP)

SPEP shows:

• M spike (sharp monoclonal peak), often in gamma region

UPEP shows:

• Bence Jones proteins (free light chains) → monoclonal spike in urine proteins

High-yield pitfall:

• In light chain–only myeloma, SPEP may be subtle/negative; UPEP or serum free light chain assay is more revealing.

Peripheral smear

• Rouleaux formation (stacked RBCs)

Bone marrow biopsy

• Clonal plasma cells, classically $\ge 10\%$
• Plasma cells may show eccentric nuclei and perinuclear hof (Golgi zone)

Imaging: why “punched out” matters

• Multiple lytic lesions (skull, vertebrae, pelvis)
• No blastic lesions (helps distinguish from prostate mets)
• Classically described as “punched-out” lesions on skull imaging

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The Must-Know Differential (because questions love “closest alternative”)

Hyperviscosity clue:

• Much more classic for Waldenström (IgM) because IgM is a large pentamer. Myeloma can increase viscosity too, but Step questions strongly associate hyperviscosity syndrome with IgM conditions.

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Treatment (what Step expects you to recognize)

Core management (conceptual)

Multiple myeloma is treated with combination therapy targeting plasma cells + supportive care.

Common regimens include:

• Proteasome inhibitors: bortezomib, carfilzomib
• Immunomodulatory drugs (IMiDs): lenalidomide, thalidomide
• Corticosteroids: dexamethasone
• Anti-CD38 monoclonal antibody: daratumumab (more Step 2-ish, but increasingly referenced)

Transplant:

• Autologous stem cell transplant is a common strategy in eligible patients (often after induction therapy).

Supportive care (extremely testable)

• Bisphosphonates (e.g., zoledronic acid) to reduce skeletal complications
• Hydration + bisphosphonates for hypercalcemia
• Vaccination and infection prophylaxis strategies may be used clinically (esp. pneumococcal)

Renal protection pearl:
Avoid nephrotoxins when possible (e.g., NSAIDs), and treat hypercalcemia aggressively.

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High-Yield Associations & Classic Board Clues

“CRAB” is the anchor

If you see monoclonal protein + any of:

• bone pain/lytic lesions
• creatinine elevation
• normocytic anemia
• hypercalcemia
  → Multiple myeloma should be at the top.

Rouleaux formation

• Stacked RBCs on smear = increased serum proteins (think myeloma)

Bence Jones proteins

• Free light chains in urine
• Associated with renal tubular damage (cast nephropathy)

AL amyloidosis connection (test favorite)

Plasma cell dyscrasias can produce light chains that deposit as amyloid:

• Restrictive cardiomyopathy
• Nephrotic syndrome
• Hepatosplenomegaly
• Macroglossia
• Shoulder pad sign (amyloid deposition)

Infections with encapsulated organisms

• Functional hypogammaglobulinemia → increased risk, especially S. pneumoniae

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Rapid-Fire USMLE-Style Mini Checkpoints

• Back pain + lytic lesions + elevated calcium → osteoclast activation from myeloma cytokines
• High total protein + low albumin-to-globulin ratio → excess monoclonal immunoglobulin
• M spike on SPEP → monoclonal gammopathy (MM, MGUS, Waldenström, etc.)
• Rouleaux on smear → high serum proteins (myeloma classic)
• Renal failure + monoclonal light chains → cast nephropathy (“myeloma kidney”)
• Hyperviscosity symptoms (blurry vision, headache, mucosal bleeding) → think Waldenström (IgM) first

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First Aid “Where This Lives” (Quick Navigation)

In First Aid (Heme/Onc), multiple myeloma is typically covered under:

• Plasma cell disorders / monoclonal gammopathies
• SPEP/UPEP interpretation
• Rouleaux formation
• Lytic bone lesions + CRAB
• AL amyloidosis associations

Use First Aid as your checklist, but make sure you can explain each CRAB feature mechanistically—that’s what turns recognition into points.

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Bottom Line (What to remember on test day)

Multiple myeloma = clonal plasma cells producing monoclonal immunoglobulin/light chains, causing:

• Bone lytic lesions (osteoclast activation)
• Renal failure (light chains + hypercalcemia)
• Anemia (marrow crowding)
• Infections (loss of functional antibodies)

When a stem hands you bone pain + renal issues + protein abnormalities, don’t just pattern-match—use CRAB + SPEP/UPEP + rouleaux to lock it in.