Which one of the following pairs of chemical substance, is correctly categorized ?

Options

A. Troponin and myosin - Complex proteins in striated muscles
B. Secretin and rhodopsin - Polypeptide hormones
C. Calcitonin and thymosin - Thyroid hormones
D. Pepsin and prolactin - Two digestive enzymes secreted in stomach

Answer

To determine which pair of chemical substances is correctly categorized, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Analyze the First Pair: Troponin and Myosin**
- Troponin is a complex of three regulatory proteins (troponin C, I, and T) that play a crucial role in muscle contraction, specifically in striated muscles (skeletal and cardiac).
- Myosin is a motor protein that interacts with actin to facilitate muscle contraction.
- **Conclusion**: This pair is correctly categorized as both are integral components of striated muscles.

2. **Analyze the Second Pair: Secretin and Rhodopsin**
- Secretin is a polypeptide hormone released by the duodenum to stimulate the secretion of digestive fluids from the pancreas and liver.
- Rhodopsin is a light-sensitive pigment found in the rods of the retina, essential for vision in low-light conditions, but it is not a hormone.
- **Conclusion**: This pair is incorrectly categorized since rhodopsin is not a hormone.

3. **Analyze the Third Pair: Calcitonin and Thymosin**
- Calcitonin is a peptide hormone produced by the thyroid gland that helps regulate calcium levels in the blood.
- Thymosin is a hormone secreted by the thymus that plays a role in the development of T cells for the immune system.
- **Conclusion**: This pair is incorrectly categorized as calcitonin is a thyroid hormone, while thymosin is not.

4. **Analyze the Fourth Pair: Pepsin and Prolactin**
- Pepsin is a digestive enzyme produced in the stomach that breaks down proteins.
- Prolactin is a hormone produced by the pituitary gland that stimulates breast development and milk production.
- **Conclusion**: This pair is incorrectly categorized as pepsin is a digestive enzyme, while prolactin is a hormone.

### Final Conclusion:
The only correctly categorized pair is **Troponin and Myosin**, as both are involved in muscle contraction in striated muscles.