## **Core Concept**
Primary lymphedema refers to a condition characterized by chronic swelling of one or more extremities due to impaired lymphatic function, without any identifiable cause. It is a congenital or idiopathic condition.

## **Why the Correct Answer is Right**
To determine the correct answer, let's evaluate each statement:
- **Prevalence**: The prevalence of primary lymphedema is actually much lower than 2%, making option a potentially incorrect without further context.
- **Classification by Age of Onset**:
- **Praecox**: Onset before the age of 35, typically during puberty.
- **Tarda**: Onset after the age of 35.
Thus, option b seems incorrect based on the definition.
Option c seems incorrect as it misclassifies the age of onset for "praecox."
- **Milroy's Disease**: This is a genetic condition that causes lymphedema, primarily of the legs, and is associated with primary lymphedema. So, option d seems correct.
- **Lymphangiosarcoma**: This is a rare complication that can occur in chronic lymphedema, known as Stewart-Treves syndrome. So, option e seems correct.

## **Why Each Wrong Option is Incorrect**
- **Option A (a)**: Prevalence being 2% seems high for primary lymphedema; the exact prevalence might vary but stating it as 2% could be misleading or incorrect without context.
- **Option B (b)**: Onset between 2-35 years does not specifically indicate "tarda"; rather, lymphedema tarda refers to onset after 35 years.
- **Option C (c)**: Onset >35 yrs indicates lymphedema tarda, not praecox. Praecox refers to onset before 35, typically during puberty.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that primary lymphedema can be associated with **Milroy's disease**, which is a hereditary condition. Additionally, a significant complication of chronic lymphedema (not limited to primary but also secondary) is the development of **lymphangiosarcoma** (Stewart-Treves syndrome).

## **Correct Answer: D.**