## **Core Concept**
The patient's symptoms and blood pressure changes with position are indicative of a condition known as **supine hypotension syndrome**. This condition is associated with pregnancy, particularly in the third trimester, and involves a significant drop in blood pressure when the woman is in the supine position.

## **Why the Correct Answer is Right**
The most likely explanation for these findings is **supine hypotension syndrome**, which occurs due to the compression of the **inferior vena cava** (IVC) by the gravid uterus when the woman is in the supine position. This compression reduces venous return to the heart, subsequently decreasing cardiac output and leading to a drop in blood pressure. The normal blood pressure when standing or sitting suggests that the issue is positional and not due to a systemic vascular or cardiac problem.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it is not specified, but generally, other causes of hypotension such as dehydration, cardiac issues, or severe infection (sepsis) would not typically present with such a positional change in blood pressure.
- **Option B:** Similarly, without specifics, any condition not directly related to the mechanical effects of the gravid uterus on major vessels would not explain the positional nature of the hypotension.
- **Option C:** This would be incorrect if it suggested a condition not directly related to the mechanical obstruction caused by the gravid uterus in the supine position.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **supine hypotension syndrome** can be alleviated by turning the patient onto her side, which relieves the compression on the IVC and restores venous return and cardiac output. This is an immediate and effective management strategy.

## **Correct Answer:** D. Supine hypotension syndrome.