**Core Concept**
Orthostatic hypotension is a condition characterized by a significant drop in blood pressure upon standing or changing positions, leading to symptoms such as faintness and nausea. This condition is exacerbated by pregnancy due to the increase in blood volume and the decrease in systemic vascular resistance.

**Why the Correct Answer is Right**
The symptoms described in the scenario are consistent with orthostatic hypotension, which is a common condition in pregnancy, particularly in the third trimester. The increase in blood volume and the decrease in systemic vascular resistance during pregnancy lead to a relative hypovolemia, which is further exacerbated by the supine position. When the patient turns on her side or gets up, she is able to increase venous return to the heart, which in turn increases cardiac output and blood pressure, relieving her symptoms. The autonomic nervous system's inability to compensate for the changes in blood pressure is also a contributing factor to this condition.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is not relevant to the symptoms described in the scenario. Supraventricular tachycardia (SVT) is a type of arrhythmia that can cause symptoms such as palpitations and shortness of breath, but it does not typically cause orthostatic hypotension.
* **Option B:** This option is not relevant to the symptoms described in the scenario. Pulmonary embolism (PE) is a condition characterized by a blockage of one of the pulmonary arteries, which can cause symptoms such as chest pain and shortness of breath, but it does not typically cause orthostatic hypotension.
* **Option C:** This option is not relevant to the symptoms described in the scenario. Hypertension is a condition characterized by high blood pressure, which is not consistent with the symptoms described in the scenario.

**Clinical Pearl / High-Yield Fact**
The supine position can exacerbate orthostatic hypotension in pregnancy due to the compression of the inferior vena cava by the uterus, which can decrease venous return to the heart and decrease cardiac output.

**Correct Answer: D. Orthostatic hypotension**