What Microvillus Inclusion Disease means for Sam, in the most basic way, is that his intestines do not work properly. His intestines do not absorb nutrients like most people and he has to be supplied with all of his nutrition through an IV. In Sam's case this is done in a central line that starts in the center of his chest, tunnels under his skin to a vein in his neck and then travels down that vein to a spot close to his heart.
In slightly more detail, the cells in the intestine are normally lined with small hair like structures called microvilli. These structures are what do the actual absorbing of nutrients in your intestines.
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A much more technical description of Sam's disease can be found here: