The sulpha drugs act on the principle of:

Options

A. competitive inhibition
B. non-competitive inhibition
C. allosteric modulation
D. feed-back inhibition

Answer

### Step-by-Step Solution:

1. **Understanding Sulfa Drugs**: Sulfa drugs are a class of antibiotics that inhibit the growth of bacteria. They are known for their ability to interfere with bacterial metabolism.

2. **Principle of Action**: The question asks about the principle on which sulfa drugs act. The options provided are:
- Competitive inhibition
- Non-competitive inhibition
- Allosteric modulation
- Feedback inhibition

3. **Identifying the Correct Principle**:
- **Competitive Inhibition**: This occurs when an inhibitor (in this case, sulfa drugs) resembles the substrate and competes for binding to the active site of the enzyme. By binding to the active site, the inhibitor prevents the actual substrate from binding, thus inhibiting the enzyme's activity.
- **Non-competitive Inhibition**: This type of inhibition does not involve competition for the active site. Instead, the inhibitor binds to a different part of the enzyme, reducing its activity regardless of whether the substrate is bound or not.
- **Allosteric Modulation**: This involves binding to a site other than the active site, causing a change in the enzyme's shape and affecting its activity. It does not directly compete with the substrate.
- **Feedback Inhibition**: This is a regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme involved in its production. This is not how sulfa drugs work.

4. **Conclusion**: Since sulfa drugs act by competing with the substrate for the active site of the enzyme involved in folic acid synthesis in bacteria, the correct answer is **competitive inhibition**.

### Final Answer:
The sulfa drugs act on the principle of **competitive inhibition**.

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