which of the following event is/are included in fast block to prevent polyspermy?

Options

A. zona reactions
B. depolarization of ovum membrane
C. cortical reaction
D. closing of Na^+ chanels of the ovum membrane

Answer

To solve the question regarding the events included in the fast block to prevent polyspermy, we will analyze the provided information step by step.

### Step 1: Understand Polyspermy
Polyspermy is the fertilization of an egg by more than one sperm. This can lead to abnormal development of the embryo, which is why mechanisms are in place to prevent it.

**Hint:** Remember that polyspermy is problematic for embryo development.

### Step 2: Identify the Mechanisms to Prevent Polyspermy
There are two main mechanisms to prevent polyspermy: the fast block and the slow block. The fast block occurs immediately after the first sperm enters the egg.

**Hint:** Focus on immediate responses that occur right after fertilization.

### Step 3: Depolarization of the Ovum Membrane
One of the key events in the fast block is the depolarization of the ovum membrane. When a sperm enters the egg, the electrical charge across the membrane changes significantly. This change in charge (from around -60 to -80 mV to about -10 to +5 mV) prevents additional sperm from fusing with the egg.

**Hint:** Think about how electrical changes can affect the ability of other sperm to enter.

### Step 4: Evaluate Other Events
- **Zona Reactions:** These occur later and involve changes to the zona pellucida, which is not part of the fast block.
- **Cortical Reaction:** This occurs in the female reproductive tract and is part of the slow block mechanism.
- **Closing of Sodium Channels:** While this is related to membrane potential, it is not the primary mechanism for the fast block.

**Hint:** Differentiate between immediate and delayed responses in the fertilization process.

### Step 5: Conclusion
The correct event included in the fast block to prevent polyspermy is the **depolarization of the ovum membrane**. This rapid change in membrane potential is crucial for stopping additional sperm from entering the egg.

**Final Answer:** The event included in the fast block to prevent polyspermy is the depolarization of the ovum membrane.