**Core Concept**
Rare earth screens are a type of intensifying screen used in radiography to enhance the quality of radiographic images. They work by converting X-ray energy into visible light, allowing for improved image quality at lower X-ray exposure levels.

**Why the Correct Answer is Right**
Rare earth screens are more efficient at converting X-ray energy into visible light compared to calcium tungstate screens. This increased efficiency allows for a decrease in patient exposure while maintaining image quality. The rare earth elements in these screens, such as gadolinium and dysprosium, have a higher Z number than calcium tungstate, enabling them to absorb and convert more X-ray energy.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because there is no evidence to suggest that rare earth screens increase patient exposure compared to calcium tungstate screens.

**Option B:** Incorrect because the question is asking for a relative decrease in patient exposure, not an absolute value.

**Option C:** Incorrect because the actual decrease in patient exposure is not specified in this option.

**Clinical Pearl / High-Yield Fact**
When selecting radiographic screens, it's essential to consider the type of X-ray examination, the patient's size and age, and the desired image quality to ensure optimal exposure levels.

**Correct Answer:** C. 50%.