In mammals, the opening of postcaval in the right atrium is guarded by

Options

A. mitral valve
B. bicuspid valve
C. thebesian valve
D. eustachian valve

Answer

To solve the question regarding what guards the opening of the postcaval vein in the right atrium of mammals, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Postcaval Vein**: The postcaval vein is also known as the inferior vena cava. It is responsible for carrying deoxygenated blood from the lower parts of the body back to the heart.

2. **Identify the Location**: The postcaval vein opens into the right atrium (or right auricle) of the heart. This is an important point because it helps us understand where the guarding structure is located.

3. **Determine the Guarding Structure**: The opening of the postcaval vein in the right atrium is protected by a structure known as a wall. The options given in the question include:
- A) Mitral wall
- B) Bicuspid wall
- C) Thebesian wall
- D) Eustachian wall

4. **Select the Correct Answer**: Among the options provided, the correct structure that guards the opening of the postcaval vein in the right atrium is the Eustachian wall. Therefore, the answer to the question is option D.

### Final Answer:
The opening of the postcaval vein in the right atrium is guarded by the Eustachian wall (Option D).

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