Angiotensin II is

Options

A. a vasoconstrictor
B. a vasodilator
C. activating adrenal medulla
D. decreasing GFR

Answer

### Step-by-Step Solution:

1. **Understanding Angiotensin II**: Angiotensin II is a hormone that plays a crucial role in regulating blood pressure and fluid balance in the body.

2. **Identifying its Function**: One of the primary functions of Angiotensin II is to act as a vasoconstrictor. This means it narrows the blood vessels, which increases blood pressure.

3. **Evaluating the Options**:
- **Option A: Vasoconstrictor** - This is correct as Angiotensin II is known to cause vasoconstriction, thereby increasing blood pressure.
- **Option B: Vasodilator** - This is incorrect. A vasodilator would widen blood vessels, which is not the function of Angiotensin II. An example of a vasodilator is nitric oxide.
- **Option C: Activating adrenal medulla** - This is also incorrect. Angiotensin II does not directly activate the adrenal medulla; instead, hormones like epinephrine and norepinephrine are responsible for that.
- **Option D: Decreasing GFR (Glomerular Filtration Rate)** - While Angiotensin II can influence GFR indirectly by triggering the release of other hormones (like antidiuretic hormone), it does not directly decrease GFR.

4. **Conclusion**: Based on the evaluation of the options, the correct answer is **Option A: Vasoconstrictor**. Angiotensin II primarily functions to constrict blood vessels and increase blood pressure.