**Core Concept**
Proprioceptors are specialized sensory receptors found in muscles, tendons, and joints that detect changes in movement, position, and tension. During exercise, proprioceptors play a crucial role in regulating breathing by sending signals to the brain, which adjusts ventilation accordingly.

**Why the Correct Answer is Right**
When exercise begins, proprioceptors in the muscles and joints are activated, sending signals to the respiratory centers in the brainstem. This activation stimulates the medullary respiratory centers, leading to an increase in ventilation. The brain interprets these signals as an increase in metabolic demand, prompting an increase in breathing rate to meet the body's oxygen requirements. Proprioceptors are particularly sensitive to changes in movement and tension, making them ideal sensors for detecting the onset of exercise.

**Why Each Wrong Option is Incorrect**
* **Option A:** Stretch receptors are primarily involved in regulating lung volume and preventing overinflation during inspiration. While they play a role in controlling breathing, they are not the primary mechanism responsible for increased ventilation at the start of exercise.
* **Option C:** Pain receptors are involved in detecting noxious stimuli and are not directly related to the regulation of breathing during exercise.
* **Option D:** T PCO (partial pressure of carbon dioxide) is a measure of the concentration of CO2 in the blood, which is an important factor in regulating breathing. However, it is not the primary stimulus for increased ventilation at the start of exercise.

**Clinical Pearl / High-Yield Fact**
The brain's respiratory centers are highly sensitive to proprioceptive input, which is why individuals with proprioceptive deficits, such as those with peripheral neuropathy, may experience breathing difficulties during exercise.

**✓ Correct Answer: B. Proprioceptors**