**Core Concept**
Pitting edema is a condition characterized by the accumulation of excess fluid in the interstitial space, leading to swelling of the affected limb. In this scenario, the patient's pitting edema is likely due to a combination of factors, including hypertension and a normal thyroid-stimulating hormone (TSH) level, which suggests that hypothyroidism is not the primary cause. Understanding the pathophysiology of edema formation is crucial in determining the underlying cause.

**Why the Correct Answer is Right**
Myxedema, a condition associated with hypothyroidism, is not a direct cause of pitting edema. While myxedema can lead to swelling due to the accumulation of mucopolysaccharides in the skin, it is not typically associated with pitting edema. In contrast, the patient's pitting edema is more likely due to an increase in hydrostatic pressure (Option A) or a decrease in colloid osmotic pressure (Option B), which can occur in patients with hypertension. Lymphatic obstruction (Option C) can also lead to pitting edema, as it prevents the proper drainage of interstitial fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in hydrostatic pressure, such as that seen in hypertension, can lead to the filtration of fluid from the capillaries into the interstitial space, resulting in pitting edema.

**Option B:** A decrease in colloid osmotic pressure, such as that seen in nephrotic syndrome, can also lead to the accumulation of fluid in the interstitial space, resulting in pitting edema.

**Option C:** Lymphatic obstruction can prevent the proper drainage of interstitial fluid, leading to the accumulation of fluid in the affected limb and resulting in pitting edema.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between pitting edema and non-pitting edema, as the underlying causes can be distinct. Pitting edema is typically associated with conditions that lead to an increase in hydrostatic pressure or a decrease in colloid osmotic pressure, whereas non-pitting edema is often associated with conditions that lead to the accumulation of mucopolysaccharides in the skin, such as myxedema.

**Correct Answer Line**
✓ Correct Answer: D. Myxedema