**Core Concept**
The use of collimators in radiography is crucial for optimizing image quality while minimizing patient exposure. Collimators are devices that shape the X-ray beam to conform to the size of the image receptor, reducing scatter radiation and improving image resolution.

**Why the Correct Answer is Right**
When a rectangular collimator is used, it reduces the patient's skin surface exposed by limiting the area of the X-ray beam. This is achieved by using a collimator with a rectangular shape that matches the dimensions of the image receptor, allowing for a more focused beam of radiation to be directed at the patient. As a result, the area of the skin surface exposed to X-rays is significantly reduced, compared to the use of a round collimator.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a clear explanation of how a rectangular collimator reduces patient exposure. While it may be true that a rectangular collimator can reduce exposure in some cases, this option lacks specificity and clarity.

**Option B:** This option is incorrect because it suggests that a rectangular collimator increases patient exposure, which is the opposite of the correct answer.

**Option C:** This option is incorrect because it implies that a rectangular collimator has no effect on patient exposure, which is not accurate.

**Clinical Pearl / High-Yield Fact**
When selecting a collimator, it's essential to consider the size and shape of the image receptor to minimize patient exposure and optimize image quality. A rectangular collimator is often used in conjunction with a digital radiography system to achieve high-quality images with reduced exposure.

**Correct Answer: D.**