## Core Concept
The amniotic fluid plays a crucial role in fetal development and protection during pregnancy. After 20 weeks of gestation, the composition and primary contributors to the amniotic fluid change.

## Why the Correct Answer is Right
The major contribution to amniotic fluid after 20 weeks of gestation comes from **fetal urine**. By this stage, fetal urine production becomes the primary source of amniotic fluid, accounting for about 90% of its production. This is a critical process as it not only contributes to the volume of amniotic fluid but also reflects fetal kidney function and overall fetal well-being.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although **fetal skin** does contribute to the amniotic fluid, especially in early gestation, its role diminishes significantly after 20 weeks.
- **Option B:** This option is incorrect as there is no commonly recognized contributor to amniotic fluid labeled simply as **"maternal plasma"** that would be a major source after 20 weeks.
- **Option C:** This option might seem plausible but is incorrect because **fetal lung fluid** does not contribute significantly to the amniotic fluid volume in the same way fetal urine does.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that decreased fetal urine production, which can be assessed via ultrasound parameters like the **amniotic fluid index (AFI)**, is associated with potential fetal growth restriction and renal anomalies. Thus, assessing amniotic fluid volume is a critical component of fetal monitoring.

## Correct Answer: D. Fetal urine.