**Core Concept**
The Barr body is a chromatin mass found in the nucleus of cells that are genetically female (46,XX). It is a visible indicator of the presence of two X chromosomes.

**Why the Correct Answer is Right**
The Barr body is formed when one of the two X chromosomes in a female cell inactivates through a process called X-chromosome inactivation. This inactivated X chromosome condenses into a dense, heterochromatic mass called a Barr body. Sex chromatin testing involves staining cells with a specific dye, such as Feulgen or Giemsa, to visualize the Barr body. The presence of a Barr body in a cell indicates that the individual is genetically female (46,XX).

**Why Each Wrong Option is Incorrect**
**Option B:** Testosterone receptors are involved in the regulation of androgen activity, not sex chromatin testing. They are not directly related to the determination of an individual's sex chromosome constitution.
**Option C:** Hormone levels, such as estrogen or testosterone, can provide information about an individual's sex but are not used for sex chromatin testing. Hormone levels can be influenced by a variety of factors and are not a reliable indicator of sex chromosome constitution.
**Option D:** Phenotypic features, such as physical characteristics or secondary sex characteristics, can provide clues about an individual's sex but are not definitive for sex chromatin testing. Some individuals with sex chromosome abnormalities may have typical phenotypic features, while others may have atypical or ambiguous features.

**Clinical Pearl / High-Yield Fact**
It's essential to note that sex chromatin testing using Barr bodies is not a definitive method for determining sex chromosome constitution. Other methods, such as karyotyping or molecular genetic testing, are more accurate and reliable.

**✓ Correct Answer: A. Barr body**