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PBM is a command, not a supply. It tells the cell to produce energy—but provides none of the materials required to carry out that task. The cell must already have sufficient oxygen, NAD⁺, and metabolic stability to generate ATP in response. When terrain is depleted—due to mitochondrial dysfunction, oxidative stress, therapeutic history, or chronic inflammatory strain—the instruction cannot be fulfilled. The signal is received, but the response fails. This paper introduces the concept of Terrain-Vulnerable Systems (TVS), a subset of the broader Terrain Integrity Model, to explain how even well-targeted biological interventions can amplify collapse when applied to systems operating below energetic threshold. PBM in this context does not fail because it is incorrect—it reveals failure that was already underway. The result is not harm by mechanism, but exposure by acceleration. The purpose of this work is to clarify the boundary between support and overreach, and to define the energetic conditions required for photobiomodulation to behave as a regenerative modality rather than an unbuffered demand signal.