(Choice B) Elastin, a fibrous protein in the connective tissue, is named for the elastic properties it imparts to skin, blood vessels, and lung alveoli. Elastin fibers can be stretched to several times their original length but will recoil when the stretching forces are withdrawn. Elastin is synthesized from the polypeptide precursor tropoelastin.
(Choice C) Fibrillin-1 is a major component of the microfibrils that form a sheath around elastin. Microfibrils are abundantly present in blood vessels and in the suspensory ligaments of the lens. Defects in the fibriffin-1 gene cause classic autosomal dominant Marfan syndrome.
(Choice D) Hyaluronic acid is another major component of the soft tissue’s extracellular matrix, including synovial fluid and skin. Exogenous injection can be used to restore viscoelasticity to the synovial fluid in osteoarthritis; softtissue fillers can also be used in patients concerned about age-related volume loss (eg, nasolabial folds).
(Choice E) Laminins are heterotrimeric glycoproteins that bind to type IV collagen underlying epithelial cells. They contribute to the organization and function of the basal lamina (basement membrane).
(Choice F) Proteoglycans are composed of glycosaminoglycans (GAGs), which provide compressibility to tissues. Patients with deficiencies in lysosomal enzymes cannot break down GAGs, resulting in mucopolysaccharidoses (eg, Hurler syndrome, Hunter syndrome) characterized by soft tissue and skeletal disease.

(Choice A) Ascorbic acid (vitamin C) is required for hydroxylation of proline and lysine residues in collagen synthesis; therefore, it plays an important role in connective tissue maintenance and wound healing.
(Choice B) Folate participates in single carbon transfer reactions, as in the de novo synthesis of purines and thymidine. Folate sup plements will improve megaloblastic anemia resulting from folate deficiency but will not improve the anemia in orotic aciduria.
(Choice C) Guanine and adenine are purine bases present in DNA and RNA. Orotic aciduria is a defect in the synthesis of pyrimidine bases, so supplementation with purines would not affect orate synthesis.
(Choice D) Iron supplementation improves iron deficiency anemia, classically a microcytic hypochromic anemia.
(Choice E) Pyridoxine (vitamin Ba) supplementation is indicated during treatment with isoniazid. Pyridoxine is a cofactor in transamination, deamination, decarboxylation, and condensation reactions.

(Choice A) Cysteine supplementation may be required in homocystinuria, a condition in which cystathionine synthase deficiency leads to reduced cysteine production and homocysteine and methionine accumulation. Fair complexion and seizures may be seen in infancy; other characteristic features (eg, downward lens dislocation, thrombosis) usually present later. However, musty body odor would not be expected.
(Choices B, C, and F) The branched-chain amino acids (isoleucine, leucine, valine) are elevated in maple syrup urine disease, which is due to branched-chain alpha-ketoacid dehydrogenase deficiency. Buildup of branchedchain amino acids and their metabolites leads to ketonuria (urine with a sweet odor) and encephalopathy within a week of birth; management is dietary restriction of these amino acids.