**Core Concept**
Testicular feminization syndrome, also known as androgen insensitivity syndrome, is a rare genetic condition where individuals are genetically male (XY) but resistant to androgens, such as testosterone. This condition leads to the development of female external genitalia despite the presence of testes.

**Why the Correct Answer is Right**
The correct answer, **D. High testosterone levels**, is due to the fact that individuals with testicular feminization syndrome have testes that produce normal levels of testosterone. However, due to the insensitivity to androgens, the body is unable to respond to the testosterone, resulting in the development of female external genitalia. The testes produce testosterone, which is then aromatized to estrogen, leading to the development of female secondary sexual characteristics. This condition is characterized by a lack of response to androgens, not a deficiency in their production.

**Why Each Wrong Option is Incorrect**
**Option A:** The XX pattern is associated with Turner syndrome, not testicular feminization syndrome.

**Option B:** Individuals with testicular feminization syndrome are typically reared as females, not males, due to the development of female external genitalia.

**Option C:** Well-formed female internal genitalia are not a feature of testicular feminization syndrome, as the internal genitalia are typically male (i.e., testes, epididymis, vas deferens, and seminal vesicles).

**Clinical Pearl / High-Yield Fact**
Testicular feminization syndrome is an X-linked recessive disorder, meaning that the gene responsible for the condition is located on the X chromosome. This condition highlights the importance of understanding the complex interplay between hormones and their receptors in human development.

**✓ Correct Answer: D. High testosterone levels**