Multiple Myeloma & MGUS

Tempting to pick fibrinogen since fibrinogen is the classic cause of rouleaux in inflammatory states. But myeloma has a specific protein causing this, not a general inflammatory response. Think of RBCs as magnets that normally repel each other because they have the same charge. M-protein is a chemical neutralizer that strips the negative charge off the surface, letting the magnets suddenly flip and stack face-to-face. Albumin works in the opposite direction (stabilizes charge repulsion). Fibrinogen is the inflammatory rouleaux cause, M-protein is the myeloma-specific cause. Correct: B.

Rouleaux formation is caused by elevated monoclonal immunoglobulin (M-protein) in the serum. Normally, red blood cells repel each other because of their negative surface charge. High concentrations of immunoglobulins and other proteins reduce the zeta potential (surface charge repulsion), allowing RBCs to stack into rouleaux. This is also why the ESR is dramatically elevated in myeloma. Albumin would have the opposite effect on ESR. Fibrinogen can contribute in other inflammatory states but the question is pointing to the M-protein as the specific cause in myeloma.

Break it down: M-protein reduces zeta potential (surface charge) on RBCs, allowing rouleaux stacking; ESR dramatically elevated in myeloma because M-protein promotes the same stacking that slows RBC sedimentation rate; albumin has the opposite effect on ESR; fibrinogen = inflammatory rouleaux, M-protein = myeloma-specific rouleaux.

Tempting to use contrast CT since imaging the spine for cord compression seems urgent enough to accept the risk. But in myeloma kidney, the tubules are not working at baseline and the contrast interaction is not just the usual risk. Think of the tubules as partially clogged pipes: Bence-Jones protein casts are already blocking them. Iodinated contrast interacts with the Tamm-Horsfall protein inside the clogged pipes, precipitating more solid material on top of the existing blockage. The pipes go from partially clogged to acutely obstructed. Correct: B.

In myeloma kidney (cast nephropathy), tubules are already partially obstructed by Bence-Jones protein (free light chain) casts. Iodinated contrast agents interact with the Tamm-Horsfall protein and precipitated light chains in the tubular lumen, forming additional obstructing casts. This can precipitate acute tubular necrosis on top of existing cast nephropathy. The combination is acutely nephrotoxic beyond the baseline contrast-induced nephropathy risk. This is a well-recognized contraindication. If cross-sectional imaging is needed, use non-contrast CT (which is actually preferred for lytic bone lesions anyway) or MRI.

Break it down: myeloma kidney = Bence-Jones protein (free light chain) casts in tubules; iodinated contrast interacts with Tamm-Horsfall protein to form additional casts; leads to acute tubular necrosis on top of cast nephropathy; use non-contrast CT or MRI instead; iodinated contrast is absolutely contraindicated in cast nephropathy.

Tempting to pick tall R waves (LVH pattern) since the walls are thick on echo. Thick walls normally mean more muscle mass which normally means more electrical amplitude. Cardiac amyloidosis is the exception that paradoxically goes in the opposite direction. Think of amyloid fibrils as an electrical insulator woven into the wall of the heart: the myocytes generate their normal signal, but the amyloid blanket absorbs it before the signal reaches the EKG leads. The wall is thick, but the signal is faint. That discordance is the fingerprint. Correct: C.

This is the pathognomonic combination: thick walls on echo + LOW voltage on EKG. Amyloid fibrils infiltrate the myocardium and do not conduct electricity effectively (they are non-conducting protein deposits). The electrical signal generated by myocytes is absorbed and dampened by the amyloid before reaching the surface EKG leads. This is the exact opposite of true left ventricular hypertrophy (which gives high voltage). The discordance between wall thickness on echo and EKG voltage is the key diagnostic clue for cardiac amyloidosis. Tall R waves (choice A) would suggest LVH, not amyloid. Delta waves (choice B) are for Wolff-Parkinson-White.

Break it down: cardiac amyloidosis = thick walls on echo PLUS low voltage on EKG (paradoxical discordance); amyloid = electrical insulator that absorbs the signal; true LVH = thick walls plus HIGH voltage; low voltage plus thick walls = amyloid until proven otherwise.

Tempting to diagnose active myeloma based on plasma cell percentage since that is the biopsy finding and it seems high enough. But the biopsy number alone is not enough without CRAB. Think of CRAB criteria as the bridge between "abnormal test" and "disease requiring treatment": the plasma cells are the production factory and the CRAB findings are evidence that the factory is actively damaging the building. No CRAB damage = the factory is running but the building is still standing. Treat MGUS with observation, not chemotherapy. Correct: C.

The single most important distinction between MGUS and active myeloma is the presence or absence of CRAB end-organ damage. This patient has none (calcium normal, creatinine normal, hemoglobin normal by marrow criteria, no bone lesions). The CRAB criteria define when a plasma cell dyscrasia becomes clinically significant and requires treatment.

Why not D? Plasma cell percentage alone is important, but it's the combination with CRAB that defines active myeloma. A patient with 15% plasma cells but no CRAB is smoldering myeloma, not active myeloma. A patient with even 11% plasma cells plus any CRAB criterion meets the diagnosis of active myeloma. CRAB is the single definitional dividing line between "watch" and "treat."

Break it down: CRAB = hyperCalcemia, Renal failure, Anemia, Bone lesions; CRAB present = active myeloma, treat; CRAB absent = MGUS or smoldering, observe; plasma cell percentage alone does not define active myeloma without end-organ damage.