Deficiency of insulin causes diabetes mellitus . Insulin is a-----------

Options

A. vitamin
B. lipid
C. hormone
D. enzyme

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question states that the deficiency of insulin causes diabetes mellitus. We need to identify what insulin is among the given options: a vitamin, a lipid, a hormone, or an enzyme.

2. **Identifying the Role of Insulin:**
Insulin plays a crucial role in regulating blood glucose levels. Its deficiency leads to hyperglycemia (high blood sugar), which is a characteristic of diabetes mellitus.

3. **Recognizing the Source of Insulin:**
Insulin is secreted by the beta cells of the pancreas, specifically from the islets of Langerhans. This indicates that insulin is produced by a specific gland in the body.

4. **Understanding the Classification of Insulin:**
Insulin is classified as a hormone. Hormones are chemical messengers that regulate various physiological processes in the body, including metabolism.

5. **Differentiating Insulin from Other Options:**
- **Vitamin:** Vitamins are organic compounds that are essential in small amounts for various bodily functions but are not hormones.
- **Lipid:** Lipids are fats and do not function as hormones.
- **Enzyme:** Enzymes are biological catalysts that speed up chemical reactions but are not classified as hormones.

6. **Conclusion:**
Based on the above analysis, insulin is best classified as a hormone. Therefore, the correct answer to the question is that insulin is a hormone.

**Final Answer:**
Insulin is a hormone.

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