**Core Concept**
Primary amenorrhea refers to the failure to initiate menstrual cycles by the age of 16 years in the presence of normal secondary sexual characteristics or by the age of 14 years in the absence of secondary sexual characteristics. This condition can result from a variety of causes, including genetic disorders, anatomical abnormalities, hormonal imbalances, and other medical conditions.

**Why the Correct Answer is Right**
Sheehan's syndrome is a rare condition characterized by postpartum pituitary necrosis, leading to panhypopituitarism. While it can cause menstrual irregularities or even secondary amenorrhea, it is not a direct cause of primary amenorrhea. This is because Sheehan's syndrome typically affects women who have already reached reproductive age and have experienced at least one menstrual cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** MRKH (Mayer-Rokitansky-Küster-Hauser) syndrome is a congenital disorder that affects the development of the vagina and uterus, leading to primary amenorrhea. It is characterized by the absence or underdevelopment of the vagina and uterus, making it a valid cause of primary amenorrhea.
**Option C:** Kallmann's syndrome is a genetic disorder that affects the development of the hypothalamus and pituitary gland, leading to delayed or absent puberty and primary amenorrhea. It is caused by mutations in the KAL1 gene, which is responsible for the development of the olfactory system and the reproductive axis.
**Option D:** Turner's syndrome is a genetic disorder that affects females, characterized by the presence of only one X chromosome (XO). It is a common cause of primary amenorrhea, as well as short stature, delayed puberty, and infertility.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with primary amenorrhea, it is essential to consider the patient's overall medical history, including any previous medical conditions or surgeries, as well as any family history of similar conditions. A thorough physical examination and laboratory testing, including karyotyping and hormonal evaluations, can help identify the underlying cause of primary amenorrhea.

**✓ Correct Answer: B. Sheehan's syndrome**