**Core Concept**
A non-functioning uterus, despite being of normal size, is a condition often associated with Asherman's syndrome or uterine factor infertility, but in the context of normal size, it can be linked to other conditions. This scenario is particularly relevant in the context of infertility and reproductive endocrinology.

**Why the Correct Answer is Right**
The correct answer is associated with Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, a congenital disorder characterized by the underdevelopment or absence of the vagina and uterus in individuals with a 46, XX karyotype. In MRKH syndrome, the uterus is usually of normal size but non-functioning due to its underdevelopment or aplasia. This condition is often diagnosed during puberty when menstrual cycles fail to initiate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the scenario of a non-functioning uterus of normal size. While Asherman's syndrome can lead to uterine scarring and non-functionality, it typically involves the presence of adhesions and is not associated with a normal-sized uterus.
**Option B:** This option is incorrect as it is not directly related to the given scenario. While polycystic ovary syndrome (PCOS) is a common cause of infertility, it is not typically associated with a non-functioning uterus of normal size.
**Option C:** This option is incorrect because it is not directly related to the given scenario. While endometriosis can lead to infertility and pelvic pain, it is not typically associated with a non-functioning uterus of normal size.

**Clinical Pearl / High-Yield Fact**
MRKH syndrome is a congenital disorder that should be considered in the differential diagnosis of primary amenorrhea and infertility in individuals with a normal-sized uterus.

**Correct Answer:** D