First step in urine formation is

Options

A. Ultrafiltration
B. Tubular secretion
C. Selective secretion
D. Tubular reabsorption

Answer

### Step-by-Step Solution:

1. **Understanding Urine Formation**: Urine formation occurs in three main steps: filtration, reabsorption, and tubular secretion.

2. **Identifying the First Step**: The first step in urine formation is **filtration**.

3. **Filtration Process**:
- Filtration takes place in the **Bowman's capsule** of the nephron.
- During this process, blood is filtered through a filtration membrane.

4. **Composition of the Filtrate**:
- The fluid that results from filtration is essentially **plasma minus proteins and formed elements** (like red blood cells and white blood cells).
- This means that the filtrate consists of water, electrolytes, and small molecules, but not larger components like proteins or blood cells.

5. **Ultrafiltration**:
- This filtration process is referred to as **ultrafiltration** because it effectively separates smaller molecules from larger ones, ensuring that proteins and formed elements are retained in the blood.

6. **Conclusion**: Therefore, the first step in urine formation is **ultrafiltration**.