Assertion : Pheniramine is used as an antihistamine.
Reason : Antimicrobials are used for treatment of malaria.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Pheniramine is used as an antihistamine."
- Pheniramine is indeed classified as an antihistamine, which is a type of medication that helps relieve allergy symptoms such as runny nose, sneezing, and itching.

2. **Understanding the Reason**:
- The reason states that "Antimicrobials are used for the treatment of malaria."
- While some antimicrobials can be used in the treatment of malaria, the term "antimicrobials" is broad and generally refers to substances that kill or inhibit the growth of microorganisms, including bacteria, viruses, fungi, and parasites. The specific drugs used for malaria are typically classified as antimalarials, not antimicrobials.

3. **Analyzing the Relationship**:
- The assertion is true because Pheniramine is indeed an antihistamine.
- The reason is misleading because it does not accurately describe the specific treatment for malaria. While some antimicrobials may have activity against malaria-causing parasites, they are not the primary treatment.

4. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer to the question is that the assertion is true but the reason is false.

### Final Answer:
The assertion is true, but the reason is false.