**Core Concept**
The peripheral oxygen sensing chemoreceptors are located in the carotid and aortic bodies and play a crucial role in detecting changes in arterial oxygen levels, pH, and carbon dioxide levels. These chemoreceptors are sensitive to changes in the ion concentration, particularly potassium (K+), which is a key component in the transduction of oxygen levels into an electrical signal.

**Why the Correct Answer is Right**
The peripheral oxygen sensing chemoreceptors are primarily sensitive to potassium ions (K+). When oxygen levels decrease, the chemoreceptor cells undergo depolarization due to the closure of potassium channels, leading to an increase in intracellular potassium concentration. This increase in potassium ions triggers the release of neurotransmitters, such as ATP and glutamate, which stimulate the afferent nerve fibers and ultimately lead to an increase in ventilation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium ions (Na+) are not directly involved in the peripheral oxygen sensing chemoreceptors. While sodium channels are present in the chemoreceptor cells, they are not the primary ions responsible for the detection of oxygen levels.
* **Option B:** Calcium ions (Ca2+) play a role in the signal transduction pathway of the peripheral oxygen sensing chemoreceptors but are not the primary ions involved in the initial detection of oxygen levels.
* **Option D:** Chloride ions (Cl-) are not directly involved in the peripheral oxygen sensing chemoreceptors.

**Clinical Pearl / High-Yield Fact**
The peripheral oxygen sensing chemoreceptors are responsible for detecting hypoxia and triggering an increase in ventilation. This is a critical mechanism for maintaining adequate oxygenation of the body's tissues, particularly during periods of high metabolic demand or in conditions of chronic hypoxemia.

**Correct Answer:** C. Potassium (K+)