Glaucoma can be caused by

Options

A. (a) Opacity of lens
B. (b) Drying of vitreous humor
C. (c) Increase in the size of eye ball
D. (d) Blockage of canal of schlemm

Answer

### Step-by-Step Solution:

1. **Understanding Glaucoma**: Glaucoma is a condition characterized by increased intraocular pressure (IOP) that can damage the optic nerve, potentially leading to blindness.

2. **Causes of Glaucoma**: The primary cause of glaucoma is the restricted drainage of aqueous humor, which is the fluid in the eye. When this fluid cannot drain properly, it accumulates, increasing pressure within the eye.

3. **Impact of Increased Eye Pressure**: The increased pressure can damage the optic nerve. Since the optic nerve is responsible for transmitting visual information from the eye to the brain, damage to it can result in vision loss or blindness.

4. **Age Factor**: Glaucoma is more common in individuals over the age of 60, indicating that age is a significant risk factor.

5. **Other Contributing Factors**: Certain medications, such as steroids, can also contribute to the development of glaucoma. Additionally, poor blood flow to the optic nerve may exacerbate the condition.

6. **Identifying the Correct Cause from Options**:
- Option A: Opacity of lens - Incorrect, as this does not cause glaucoma.
- Option B: Drying of vitreous humor - Incorrect, since glaucoma involves fluid accumulation, not drying.
- Option C: Increase in the size of the eyeball - Incorrect, as this is a symptom rather than a cause.
- Option D: Blockage of the canal of Schlemm - Correct, as this blockage prevents proper drainage of aqueous humor, leading to increased eye pressure.

7. **Conclusion**: The correct answer to the question "Glaucoma can be caused by" is **blockage of the canal of Schlemm**.