**Core Concept**
The condition described is a classic manifestation of supine hypotension syndrome, which occurs in pregnant women due to the compression of the inferior vena cava (IVC) by the uterus. This compression leads to a decrease in venous return and subsequently a drop in blood pressure.

**Why the Correct Answer is Right**
When a pregnant woman lies in a supine position, the enlarged uterus can compress the IVC, restricting blood flow back to the heart. This reduction in venous return decreases cardiac output, leading to a decrease in blood pressure. The compression of the IVC is most pronounced when the woman is in a supine position, as gravity exacerbates the compression of the IVC by the uterus.

**Why Each Wrong Option is Incorrect**
* **Option A:** Orthostatic hypotension is characterized by a drop in blood pressure upon standing, not supine positioning.
* **Option B:** Preeclampsia is a condition characterized by high blood pressure and proteinuria in pregnancy, not a drop in blood pressure in a supine position.
* **Option C:** Cardiac tamponade is a condition where fluid accumulates in the pericardial space, leading to compression of the heart and decreased cardiac output, but it is not related to the compression of the IVC by the uterus.

**Clinical Pearl / High-Yield Fact**
The Heimlich maneuver, used to relieve choking, can also be used to alleviate supine hypotension syndrome by elevating the woman's legs and moving her to a left lateral decubitus position, which helps to alleviate compression of the IVC by the uterus.

**Correct Answer: C. Cardiac tamponade is a condition where fluid accumulates in the pericardial space, leading to compression of the heart and decreased cardiac output, but it is not related to the compression of the IVC by the uterus.