**Core Concept**
The question requires an understanding of the role of androgens in female sexual development and the consequences of androgen receptor defects. Androgens, such as testosterone, play a crucial role in the development of secondary sexual characteristics, including body hair, in females. Androgen receptor defects can lead to impaired androgen action, resulting in abnormalities in sexual development.

**Why the Correct Answer is Right**
The correct answer is an androgen receptor defect because the patient has an XY chromosome pattern, indicating she has testes that produce androgens. However, she lacks body hair, a characteristic that would normally be present in females with functioning androgens. This suggests that the androgen receptors in her body are not responding to the androgens produced by her testes. As a result, she has impaired androgen action, leading to the absence of body hair and primary amenorrhea.

**Why Each Wrong Option is Incorrect**

* **Option A:** An estrogen receptor defect would lead to abnormalities in estrogen-mediated processes, such as breast development, which is normal in this patient. Estrogen receptors are not involved in the development of body hair.
* **Option B:** Excess hormone production would not explain the absence of body hair, as the patient's androgen levels are likely normal. The issue is not the amount of androgens produced, but rather the impaired response to these hormones due to the androgen receptor defect.
* **Option D:** Decreased hormone production would not explain the patient's phenotypic pattern, as she has normal breast development and female sex characteristics. The issue is not the lack of androgens, but rather the impaired response to these hormones.

**Clinical Pearl / High-Yield Fact**
Androgen insensitivity syndrome (AIS) is a condition characterized by an androgen receptor defect, leading to impaired androgen action. Patients with AIS have an XY chromosome pattern, but develop female external genitalia and lack secondary sexual characteristics, including body hair. This condition is a classic example of a genetic disorder affecting hormone action.

✓ Correct Answer: C. androgen receptor defect