Which of the following blocks the entry of additional sperm, once a single sperm cell encounters ova?

Options

A. Corpus luteum
B. Plasma membrane
C. Corona radiata
D. Zona pellucida

Answer

To solve the question "Which of the following blocks the entry of additional sperm once a single sperm cell encounters ova?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which structure prevents additional sperm from entering the egg after the first sperm has successfully fertilized it.

2. **Identifying the Options**: The options given are:
- A. Corona radiata
- B. Plasma membrane
- C. Corpus luteum
- D. Zona pellucida

3. **Analyzing Each Option**:
- **A. Corona radiata**: This is a layer of cells surrounding the egg that provides protection but does not block additional sperm entry after fertilization.
- **B. Plasma membrane**: The plasma membrane is semi-permeable and does not specifically block additional sperm; it is involved in the overall structure of the cell.
- **C. Corpus luteum**: This is a temporary endocrine structure involved in the menstrual cycle and does not play a role in blocking sperm entry.
- **D. Zona pellucida**: This is a glycoprotein layer surrounding the plasma membrane of the oocyte. Once a sperm penetrates the zona pellucida, it triggers a reaction that prevents other sperm from entering, effectively blocking additional sperm.

4. **Conclusion**: Based on the analysis, the structure that blocks the entry of additional sperm after one sperm has fertilized the egg is the **Zona pellucida**.

### Final Answer:
The correct option is **D. Zona pellucida**.

---