Ans. C. Active TransportFree amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side-chain.Transporters of Amino AcidsFor Neutral Amino acidsFor Basic Amino acids and Cysteine.For Imino Acids and GlycineFor Acidic Amino acidsFor Beta Amino Acids (Beta-Alanine)Meister's CycleFor absorption of Neutral Amino acids from Intestines, Kidney tubules and brain.The main role is played by Glutathione (GSH).For transport of 1 amino acid and regeneration of GSH 3 ATPs are required.Disorders associated with Meister's Cycle Oxoprolinuria5 Oxoprolinase deficiency leads to OxoprolinuriaDisorders associated with absorption of amino acidsHartnup's DiseaseMalabsorption of neutral amino acids, including the essential amino acid tryptophan SLC6A19, which is the major luminal sodium-dependent neutral amino acid transporter of small intestine and renal tubules, has been identified as the defective proteinBlue Diaper Syndrome or Drummond Syndrome IndicanuriaTryptophan is specifically malabsorbed and the defect is expressed only in the intestine and not in the kidney. Intestinal bacteria convert the unabsorbed tryptophan to indican, which is responsible for the bluish discoloration of the urine after its hydrolysis and oxidationCystinuriaDibasic amino acids, including cystine, ornithine, lysine, and arginine are taken up by the Na-independent SLC3A1/ SLC7A9, in the apical membrane which is defective incystinuri(a).Most common disorder associated with Amino acid malabsorptionLysinuric Protein Intolerance(SLC7A7) carrier at the basolateral membrane of the intestinal and renal epithelium is affected, with failure to deliver cytosolic dibasic cationic amino acids into the paracellular space in exchange for Na+ and neutral amino acidsOasthouse Urine Disease (Smith Strang Disease)A methionine-preferring transporter in the small intestine was suggested to be affected. Cabbage-like odor, containing 2-hydroxybutyric acid, valine, and leucineIminoglycinuriaMalabsorption of proline, hydroxyproline, and glycine due to the proton amino acid transporter SLC36A2 defectDicarboxylic AciduriaExcitatory amino acid carrier SLC1A1 is affected.Associated with neurologic symptoms such as POLIP (polyneuropathy, ophthalmoplegia, leukoencephalopathy, intestinal pseudo-obstruction