The stacking of red blood cells that occurs at low velocities is called what?

• A. Rayleigh scattering
• B. Rouleaux formation
• C. Huygens principle
• D. Parabolic flow

Answer: (B)

Rouleaux formation is the phenomenon at play when red blood cells stack together in long, coin-like rows under low shear, or low velocity, conditions. This happens because higher levels of certain plasma proteins, especially fibrinogen and globulins, reduce the negative charge that normally keeps red cells apart and promote adhesion between adjacent cells. When the flow is slow, these stacks become stable and can settle more quickly, which is part of why the erythrocyte sedimentation rate rises in states with rouleaux. Clinically, you’ll see rouleaux in inflammatory conditions, infections, and some plasma cell disorders like multiple myeloma.

Rayleigh scattering is about light scattering by particles smaller than the light’s wavelength, not cell stacking. Huygens principle is a wavefront-based idea about how waves propagate. Parabolic flow describes the velocity profile of laminar flow in pipes, not the interaction between red cells.