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The sensation of fullness in the chest accompanied by difficulty breathing is most accurately characterized by phlegm-heat obstructing the lung. In Traditional Chinese Medicine (TCM), this condition describes a scenario where there is an accumulation of phlegm due to excessive heat in the lungs, leading to symptoms such as chest tightness, coughing, and labored breathing. The presence of phlegm obstructs the normal flow of qi (energy) and can cause a feeling of heaviness and fullness in the chest.

In contrast, heart yang deficiency primarily leads to symptoms like palpitations, fatigue, and cold extremities due to insufficient warmth and energy to support cardiac function. Spleen Qi deficiency can cause digestive issues, fatigue, and a sense of heaviness but does not directly lead to chest fullness and breathing difficulties. Kidney yang deficiency is associated with various signs including lower back pain and cold sensations related to kidney health, but it is not typically characterized by a sensation of fullness in the chest as seen in phlegm-heat obstructing the lung.

Therefore, the identification of phlegm-heat obstructing the lung aligns most closely with the symptoms described, making it the correct condition for this case.