A tissue that cannot complete a pathway does not simply fail in isolation. It changes what it takes up and what it releases, and other tissues respond to those changes. This inter-organ exchange is how a local lesion becomes a whole-body phenomenon.
Three compensatory patterns are common. First, the affected tissue increases uptake of an alternative fuel that can bypass the block, if one is available and the tissue can use it. Second, the tissue releases the intermediates that accumulate upstream of the block into the bloodstream, shifting the load to other organs that can metabolize or excrete them. Third, other tissues increase their own use of the pathway's product or increase production of it, partially compensating for the deficit.
The liver and the kidney are frequent participants in these exchanges because they have broad metabolic capacity and are positioned to receive and process circulating metabolites. The observable whole-body symptoms — fatigue, altered blood metabolite levels, and organ-specific dysfunction — are the summed result of the failing pathway in the tissues that depend on it plus the compensatory responses of the tissues that try to compensate. A symptom is therefore not a direct readout of the missing enzyme; it is the end of a chain that runs from the blocked step through tissue-specific dependence and inter-organ exchange to the whole-body level.