**Core Concept**
Secondary amenorrhea refers to the cessation of menstrual bleeding in a woman who has previously menstruated. It can be caused by various factors, including hormonal imbalances, anatomical abnormalities, and systemic diseases. In this case, the patient has a history of abortion and is presenting with secondary amenorrhea, which warrants further investigation.

**Why the Correct Answer is Right**
The patient's FSH level of 6 mIU/ml is within the normal range, indicating that her ovaries are functioning properly. This makes a diagnosis of primary ovarian insufficiency (POI) or premature menopause less likely. The patient's presentation of secondary amenorrhea 6 months after an abortion suggests that the cause may be related to the pregnancy or the postpartum period. The most probable diagnosis is postpartum pituitary apoplexy or a pituitary tumor causing secondary amenorrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** Polycystic Ovary Syndrome (PCOS) is a common cause of secondary amenorrhea, but it would typically be associated with elevated FSH levels, which is not the case in this patient.
**Option B:** Thyroid disease can cause secondary amenorrhea, but there is no information provided to suggest thyroid dysfunction.
**Option C:** Adrenal insufficiency can cause secondary amenorrhea, but it is not suggested by the patient's history or FSH level.

**Clinical Pearl / High-Yield Fact**
In a postpartum patient presenting with secondary amenorrhea, consider the possibility of postpartum pituitary apoplexy or a pituitary tumor, even in the absence of other symptoms.

**Correct Answer:** D.