Rayleigh scattering is most likely to occur when encountering which of the following?

Rayleigh scattering is a phenomenon that occurs when the size of the scatterer is much smaller than the wavelength of the incident wave. In the context of ultrasound and biological tissues, red blood cells (RBCs) are particularly important due to their size and the fact that they are often smaller than the wavelengths of ultrasound waves used in diagnostic sonography.

When ultrasound waves encounter red blood cells, they scatter in various directions. This scattering can enhance the visualization of blood flow and can be particularly useful in Doppler ultrasound assessments, where the movement of RBCs is critical for evaluating hemodynamics. The unique properties of this type of scattering mean that Rayleigh scattering significantly contributes to the ultrasound images we obtain of blood vessels and flowing blood.

The other options—liver, pleura, and diaphragm—represent larger structures or interfaces that do not interact with ultrasound waves in the manner that red blood cells do. These tissues may reflect or absorb ultrasound differently and are generally not associated with Rayleigh scattering because their size is comparable to or larger than the ultrasound wavelength.