Okay, let's tackle this question. The patient is a primigravida at full term experiencing faintness when lying down but feels better when she turns to the side or sits. The key here is to think about the physiological changes in late pregnancy.

First, when a pregnant woman lies on her back, especially in the third trimester, the growing uterus can compress the inferior vena cava. This is called supine hypotension syndrome. The compression reduces venous return to the heart, leading to decreased cardiac output and subsequently lower blood pressure. The patient might feel lightheaded or faint because of reduced blood flow to the brain.

Now, when she turns to her side, particularly the left side, the pressure on the inferior vena cava is relieved. The left lateral position allows the uterus to shift off the vena cava, improving venous return and thus increasing cardiac output. This explains why she feels better in that position.

Looking at the options, the correct answer should relate to the compression of the inferior vena cava. Let's assume the options are something like:

A. Compression of aorta
B. Compression of inferior vena cava
C. Compression of ureters
D. Compression of common iliac arteries

Option B would be correct. The other options are incorrect because compression of the aorta would affect arterial flow, but the main issue here is venous return. Ureters and iliac arteries aren't directly related to this symptom.

The clinical pearl here is that supine hypotension is a classic sign in late pregnancy, managed by positioning the patient on her left side. This is a high-yield fact for exams.

**Core Concept**
This question tests the pathophysiology of **supine hypotension syndrome**, a common complication in late pregnancy. It involves **inferior vena cava compression** by the gravid uterus, reducing venous return and causing hypotension when the patient lies supine.

**Why the Correct Answer is Right**
In the third trimester, the enlarged uterus compresses the **inferior vena cava** when the patient lies supine, decreasing venous return to the heart. This reduces cardiac output and cerebral perfusion, causing faintness. Turning to the left or sitting relieves the compression, restoring venous flow and alleviating symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Compression of the aorta would impair arterial supply to the uterus and lower limbs, not cause faintness.
**Option C:** Ureteral compression causes hydronephrosis or urinary symptoms, not hypotension.
**Option D:** Common iliac artery compression is rare and unrelated to positional faintness.

**Clinical Pearl / High-Yield Fact**
**Supine hypotension syndrome** is a classic exam trap. Always consider positional hypotension in pregnant patients with symptoms in the supine position. **Left lateral decubitus positioning** is the immediate management.

**Correct Answer: B. Compression of inferior vena cava**

✓ Correct Answer: B. IVC compression