The shivering in malaria occurs when :
(a) merozoites are liberated from RBC with toxin
(b) when schizonts enter the RBC
(c) when the signet ring is formed
(d) when the schizonts are on reticulo-endothelial cells

Options

A. merozoites are liberated from RBC with toxin
B. when schizonts enter the RBC
C. when the signet ring is formed
D. when the schizonts are on reticulo-endothelial cells

Explanation

To solve the question regarding when shivering occurs in malaria, we need to analyze each option provided and understand the life cycle of the malaria-causing parasite, Plasmodium falciparum.

### Step-by-step Solution:

1. **Understanding Malaria**: Malaria is caused by the protozoan parasite Plasmodium falciparum, which is transmitted to humans through the bite of an infected female Anopheles mosquito.

2. **Life Cycle of Plasmodium**:
- After the mosquito bite, sporozoites enter the human bloodstream and travel to the liver.
- In the liver, sporozoites develop into merozoites, which are then released into the bloodstream.

3. **Merozoites and RBCs**:
- Once in the bloodstream, merozoites invade red blood cells (RBCs) and multiply within them, forming a stage called trophozoites.
- The trophozoites eventually develop into schizonts, which mature and then burst the RBCs, releasing new merozoites back into the bloodstream.

4. **Release of Toxins**:
- The bursting of the RBCs releases a toxin called hemozoin, which is responsible for the symptoms of malaria, including fever and chills (shivering).

5. **Analyzing the Options**:
- **Option (a)**: Merozoites are liberated from RBC with toxin. This is correct because the release of merozoites from RBCs does lead to the release of hemozoin, causing shivering.
- **Option (b)**: When schizonts enter the RBC. This is incorrect as the entry of schizonts does not cause shivering.
- **Option (c)**: When the signet ring is formed. This is also incorrect as the formation of the signet ring stage does not directly relate to shivering.
- **Option (d)**: When the schizonts are on reticulo-endothelial cells. This option is incorrect as it does not relate to the shivering symptom.

6. **Conclusion**: The correct answer is **(a) merozoites are liberated from RBC with toxin**.