Which of the following is NOT an extra fu organ?

The stomach is classified as a regular zang-fu organ in Traditional Chinese Medicine (TCM), specifically a fu organ that plays a crucial role in digestion and the transformation of food into qi and blood. Unlike the extra fu organs, which have special functions and are not considered part of the main organ system, the stomach is integral to the overall functioning of the body's digestive system.

Extra fu organs, such as the vessels, uterus, and brain, are understood to hold particular significance in TCM because they possess unique characteristics and play vital roles beyond the conventional understanding of organ systems. For instance, the vessels are critical for the circulation of blood and qi, the uterus is central in reproductive health, and the brain is linked with mental and neurological processes.

In summary, since the stomach is a standard fu organ essential for digestion, it is not considered an extra fu organ, setting it apart from others listed in the question.