**Core Concept**
Non-pitting edema refers to a type of edema where the skin does not indent when pressed, indicating a deeper fluid accumulation. This condition is often associated with underlying lymphatic or vascular pathology.

**Why the Correct Answer is Right**
Non-pitting edema is commonly seen in conditions that affect the lymphatic system, such as lymphedema. In lymphedema, the lymphatic vessels are impaired, leading to a buildup of protein-rich fluid in the interstitial tissue. This fluid accumulation causes the skin to become thickened and fibrotic, resulting in a non-pitting edema. The lack of pitting is due to the increased interstitial pressure and the presence of collagen fibers in the skin.

**Why Each Wrong Option is Incorrect**
**Option A:** Congestive heart failure is typically associated with pitting edema due to the increased hydrostatic pressure in the peripheral veins.
**Option B:** Cellulitis is an infection of the skin and subcutaneous tissue, which may present with pitting edema, but it is not a characteristic feature.
**Option C:** Myxedema is a condition associated with hypothyroidism, which can present with pitting edema, particularly in the face and hands.

**Clinical Pearl / High-Yield Fact**
When assessing edema, it is essential to differentiate between pitting and non-pitting edema, as the underlying cause and management strategy may differ significantly.

**Correct Answer: D. Lymphedema**