Which event ensures that only one sperm can fertilize an ovum ?

Options

A. Spermatogenesis Amphimixis
B. Amphimixis
C. Depolasation of oocyte plasma membrane
D. Capacitation

Answer

To solve the question "Which event ensures that only one sperm can fertilize an ovum?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Polyspermy**:
- Polyspermy is the condition where more than one sperm fertilizes an ovum. This is not desirable as it can lead to abnormal development.

**Hint**: Remember that polyspermy refers to multiple sperms fertilizing the ovum, which is not ideal for normal development.

2. **Mechanisms to Prevent Polyspermy**:
- The body has mechanisms to prevent polyspermy. There are two primary mechanisms:
- Fast-acting response
- Cortical reaction

**Hint**: Focus on the two main mechanisms that prevent multiple sperms from fertilizing the ovum.

3. **Fast-Acting Response**:
- When the first sperm penetrates the ovum, the plasma membrane of the ovum undergoes a change (shedding). This prevents other sperms from entering.

**Hint**: The first sperm's entry triggers an immediate change in the ovum's membrane.

4. **Cortical Reaction**:
- This reaction occurs shortly after the first sperm enters. Calcium channels open, leading to the release of cortical granules that modify the zona pellucida (the outer layer of the ovum). This change makes it impenetrable to other sperms.

**Hint**: The cortical reaction is crucial and involves calcium ions that help seal the ovum after the first sperm enters.

5. **Identifying the Correct Event**:
- Among the options provided, the event that specifically ensures that only one sperm can fertilize an ovum is the **depolarization of the oocyte plasma membrane**. This is part of the fast-acting response.

**Hint**: Look for the option that mentions changes in the plasma membrane of the oocyte as a result of sperm entry.

6. **Eliminating Incorrect Options**:
- Other options like spermatogenesis, capacitation, and amphimixes do not directly prevent polyspermy. They are related to sperm formation or fertilization but do not address the prevention of multiple sperm entry.

**Hint**: Analyze each option carefully to see if it directly relates to preventing multiple sperm from fertilizing the ovum.

### Conclusion:
The correct answer to the question is the **depolarization of the oocyte plasma membrane**, which is a mechanism that ensures only one sperm can fertilize an ovum.