The most common cause of hypoxia is inadequate oxygen delivery to tissues, which occurs when oxygen supply via the blood is insufficient to meet cellular demands. This can result from conditions like anemia, low cardiac output, or impaired oxygenation in the lungs. Without adequate oxygen delivery, tissues cannot perform aerobic metabolism efficiently, leading to cellular dysfunction and organ damage. Other options do not directly reduce oxygen availability or cause true hypoxia. Understanding this is crucial for prompt recognition and treatment in clinical practice.

Reference: Harrison's Principles of Internal Medicine, Dennis L. Kasper, 20th Edition.