According to one of the theories of aging, the decline and disappearance of which gland by late middle age is the primary
cause of aging ?

Options

A. Thyroid
B. Parathyroid
C. Thymus
D. Posterior lobe of pituitary.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks which gland's decline and disappearance is considered a primary cause of aging according to one of the theories of aging.

2. **Analyzing the Options**: We have four options to consider: thyroid gland, parathyroid gland, thymus gland, and posterior lobe of pituitary.

3. **Evaluating the Thyroid Gland**: The thyroid gland is responsible for producing thyroxin, a hormone that regulates metabolism. It does not have a direct link to the aging process, so this option can be eliminated.

4. **Evaluating the Parathyroid Gland**: The parathyroid glands regulate calcium levels in the blood through the secretion of parathormone. Similar to the thyroid, it does not play a significant role in the aging process, so this option is also eliminated.

5. **Evaluating the Thymus Gland**: The thymus gland is crucial for the development of the immune system, particularly in the maturation of T-cells. This gland is most active during childhood and puberty but shrinks significantly with age, becoming much smaller by late middle age. This decline is associated with the aging process, making this option a strong candidate.

6. **Evaluating the Posterior Lobe of Pituitary**: The posterior lobe of the pituitary gland secretes hormones like oxytocin and vasopressin, which are important for various bodily functions but are not directly linked to the aging process.

7. **Conclusion**: Based on the evaluations, the thymus gland is the gland whose decline and disappearance by late middle age is considered a primary cause of aging. Therefore, the correct answer is the **thymus gland**.

**Final Answer**: Thymus gland

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