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The *Treatise on Differences in Yin and Yang* states: "Obstruction of the Two Yangs is called Xiao (Wasting-Thirst)." The *Essential Secrets of the Pulse Essentials* states: "Dan heat transforms into Xiao Zhong (Middle Wasting-Thirst)." The "Two Yangs" refer to the Yangming Meridians. The Hand-Yangming Large Intestine Meridian governs body fluids; when suffering from Xiao, symptoms such as yellow eyes and dry mouth indicate a deficiency of fluids. The Foot-Yangming Stomach Meridian governs blood; excessive heat leads to rapid digestion and hunger, while latent fire in the blood causes blood deficiency. "Obstruction" implies insufficient body fluids that fail to moisten, all resulting from dryness-heat pathologies. This arises from long-term consumption of sweet, delicious, and fatty foods, causing Qi to overflow upward and transform into Xiao (Wasting-Thirst). Treatment should employ Lan Cao (Pei Lan, *Herba Menthae Haplocalycis*) to eliminate stale Qi. One must not consume rich meats or aromatic herbs and mineral drugs, as their fierce nature can aggravate dryness-heat.

The *Nan Jing* (Classic of Difficult Issues) states: "When pathogens reside in the Six Fu Organs, the Yang pulses are discordant." Discordance in Yang pulses leads to Qi stagnation; Qi stagnation causes the Yang pulses to become vigorous. When Yang pulses are excessively vigorous, Yin Qi fails to nourish the body, resulting in emaciation of skin and muscles. The Classics state: "In treating Xiao Dan (Wasting-Thirst), Pu Ji (Sudden Collapse), Pian Gu (Hemiplegia), Wei Jue (Atrophy and Cold Extremities), and Qi Man Fa Ni (Fullness and Rebellious Qi), these conditions often affect wealthy individuals and are diseases caused by rich diets." Qibo stated: "If the pulse is full and the disease has lasted long, it is treatable; if the pulse is wiry and small and the disease has lasted long, it is difficult to treat."

Later, Xiao was categorized into Three Xiaos:
Upper Xiao (Gao Xiao): Red cracks on the tongue, extreme thirst with desire to drink. The *Treatise on Counterflow and Regulation* states: "Heart heat transfers to the Lung, transforming into Ge Xiao (Diaphragm Wasting-Thirst)." It is treated with Bai Hu Jia Ren Shen Tang (White Tiger Decoction plus Ginseng).
Middle Xiao: Polyphagia with emaciation, spontaneous sweating, hard stools, and frequent urination. Wang Shuhe stated: "Dry mouth, excessive drinking, polyphagia but weak muscles, where Dan heat forms Xiao Zhong." It is treated with Tiao Wei Cheng Qi Tang (Regulate the Stomach Order the Qi Decoction) and San Huang Wan (Three Yellows Pill).
Lower Xiao (Shen Xiao): Irritability with thirst, dried and scorched earlobes, urine resembling paste. Wang Shuhe stated: "Scorched irritability, fluid depletion easily occurs; this is Kidney Xiao." It is treated with Liu Wei Di Huang Wan (Six-Ingredient Rehmannia Pill).

The *Zong Lu* records: If the condition progresses to the terminal stage, those who can eat will inevitably develop brain abscesses or back sores; those who cannot eat will inevitably develop abdominal fullness and distension. These are considered incurable. Elder Jiegu treated them separately: for those who can eat and feel thirsty, use Bai Hu Jia Ren Shen Tang; for those who cannot eat and feel thirsty, use Qian Shi's Fang Zi San (Bai Zhu Powder) with added Ge Gen (Kudzu Root). Once Upper and Middle Xiao are resolved, transmission to Lower Xiao ceases. The previous masters' use of medicine was indeed profound.

One might ask: "Why did it not progress to sores?" This is due to victorious Fire pathogens. The sores are painful but do not suppurate, or they discharge red fluid. The Classics state: "Having form but no pain belongs to the Yang category." One should urgently attack the Yang aspect, not the Yin aspect. Treatment focuses on the Lower Jiao; survival depends on strong Yuan Qi, while death follows if Yuan Qi is weak.

Why did it not progress to abdominal fullness? Using cold to treat heat follows standard medical principles, which cannot be discarded. However, Zang-Fu organs have varying distances from the surface: Heart and Lung are proximal, requiring smaller doses; Liver and Kidney are distal, requiring larger doses. The dosage must ensure the medicine reaches the target site. Therefore, both excess and deficiency in dosage punish healthy tissues. For instance, in Upper and Middle Xiao, if the prescription is too rapid or potent, exceeding the pathological site, it may eventually cause abdominal fullness. This is precisely what is meant by "upper heat not yet removed, middle cold newly generated." This is not the fault of the medicine, but a failure in balancing speed and moderation. Prescribing requires careful consideration.