## **Core Concept**
Normal intraocular pressure (IOP) is crucial for maintaining the shape of the eye and ensuring proper vision. It is regulated by the balance between the production of aqueous humor by the ciliary body and its drainage through the trabecular meshwork. The normal range of IOP is essential for preventing conditions such as glaucoma.

## **Why the Correct Answer is Right**
The correct answer, **10-21 mmHg**, is considered the normal range for intraocular pressure. This range is crucial because pressures above this range can lead to glaucoma, a condition that can cause optic nerve damage and vision loss if not treated. The production and drainage of aqueous humor are tightly regulated to maintain IOP within this normal range.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Pressures significantly lower than 10 mmHg can occur in conditions like ocular hypotony, which may result from trauma, surgery, or other causes, and can lead to vision problems.
- **Option B:** While closer, if it suggests a range that does not align with 10-21 mmHg, it would be incorrect based on the specific values provided.
- **Option D:** Pressures higher than 21 mmHg are considered elevated and can be associated with ocular hypertension and an increased risk of developing glaucoma.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the average intraocular pressure is around **16 mmHg**, with a standard deviation of about 3 mmHg. Pressures consistently above 21 mmHg or below 10 mmHg can be concerning and may require medical intervention. It's also important to note that some individuals can have pressures outside this range without developing glaucoma or ocular hypertension, a concept known as "ocular hypertension" or "normal-tension glaucoma."

## **Correct Answer:** . **10-21 mmHg**