**Core Concept**
Cells most sensitive to hypoxia are those that have a high demand for oxygen, rely heavily on oxidative phosphorylation for energy production, and have limited ability to adapt to low oxygen levels. This sensitivity is due to the critical role of oxygen in the electron transport chain, where it is used to generate ATP.

**Why the Correct Answer is Right**
The correct answer is neurons, which are highly sensitive to hypoxia due to their high metabolic rate and reliance on oxidative phosphorylation for energy production. Neurons have limited ability to adapt to low oxygen levels and are unable to switch to anaerobic metabolism, making them particularly vulnerable to hypoxia. This sensitivity is reflected in the clinical phenomenon of neuronal damage or death in conditions such as stroke or ischemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Red blood cells are not sensitive to hypoxia, as they lack mitochondria and rely on anaerobic metabolism for energy production.
**Option B:** Muscle cells are sensitive to hypoxia, but to a lesser extent than neurons, as they have some ability to adapt to low oxygen levels through anaerobic metabolism.
**Option C:** Epithelial cells are not particularly sensitive to hypoxia, as they have a lower metabolic rate than neurons and can adapt to low oxygen levels through various mechanisms.

**Clinical Pearl / High-Yield Fact**
In clinical practice, the sensitivity of neurons to hypoxia is reflected in the use of hypoxia as a diagnostic tool for conditions such as stroke or brain death. The presence of hypoxia can be detected through various imaging modalities, including MRI or CT scans.

**Correct Answer: C. Neurons.**