Which one of the following stages in the life-history of liver fluke infects the primary host ?

Options

A. Miracidium
B. Redia
C. Cercaria
D. Metacercaria

Answer

To determine which stage in the life-history of the liver fluke infects the primary host, we need to analyze the different stages of the liver fluke's life cycle and identify the one that directly invades the definitive host.

### Step-by-Step Solution:

1. **Understanding the Terms**:
- The primary host (or definitive host) is the organism in which the parasite reaches maturity and reproduces. For liver flukes, this is typically humans, sheep, or cattle.

2. **Identifying the Stages**:
- The life cycle of the liver fluke includes several stages:
- **Miracidium**: A ciliated larval stage that hatches from the egg and invades the first intermediate host (usually a snail).
- **Sporocyst**: This stage develops within the snail and produces more larvae.
- **Cercaria**: This is a larval stage that emerges from the snail but is still not infective to the definitive host.
- **Metacercaria**: This is the infective stage that leaves the snail and can invade the definitive host.

3. **Analyzing Each Stage**:
- **Miracidium**: Infects the snail, not the primary host.
- **Sporocyst**: Develops within the snail, not infecting the primary host.
- **Cercaria**: Exits the snail but does not infect the primary host directly.
- **Metacercaria**: This is the stage that is infective and can invade the primary host (humans, sheep, or cattle).

4. **Conclusion**:
- The stage that infects the primary host (definitive host) is the **Metacercaria**.

### Final Answer:
The stage in the life-history of the liver fluke that infects the primary host is the **Metacercaria**.

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