**Core Concept**
Nephelometry is a technique used to measure the concentration of particles or molecules in a solution, such as autoantibodies, by detecting the amount of light scattered by these particles. This principle is based on the scattering of light, which is an important phenomenon in physics and is described by the Rayleigh scattering equation.

**Why the Correct Answer is Right**
The correct answer, **A. Light attenuated in intensity by scattering**, is based on the principle that when light passes through a solution containing particles, the particles scatter the light in different directions. This scattering of light results in a decrease in the intensity of the light beam, which is then measured to determine the concentration of particles in the solution. The scattered light is detected at an angle, typically 90 degrees, to the incident light beam. This principle is the basis for nephelometry, which is widely used in clinical laboratories to detect autoantibodies and other analytes.

**Why Each Wrong Option is Incorrect**
**Option B:** Refraction of light is the bending of light as it passes from one medium to another, which is not the principle behind nephelometry.
**Option C:** Reduced transmission of light refers to the decrease in the amount of light that passes through a solution, which is not the specific principle used in nephelometry.
**Option D:** Filtration of solutes by kidney is a process that occurs in the kidneys to remove waste products from the blood, which is not related to the principle of nephelometry.

**Clinical Pearl / High-Yield Fact**
Nephelometry is a sensitive technique for detecting autoantibodies, but it can be affected by the presence of lipids or other substances that can interfere with the light scattering. It is essential to use a suitable calibration curve and to ensure that the sample is free of interfering substances to obtain accurate results.

**✓ Correct Answer: A. Light attenuated in intensity by scattering**