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Diet profoundly affects many inborn errors of metabolism. In some cases, treatment involves restricting the supply of metabolites preceding an enzymatic defect. Phenylalanine restriction, for example, limits neurotoxicity in phenylketonuria. Diets are also supplemented with enzyme cofactors to bolster residual enzyme activity. Hence, biotin is employed in propionic acidemia to prevent mitochondrial toxicity of propionyl-CoA. This case is a cautionary tale of diet masking detection of an inborn metabolic error.

As is typical in a sick infant, the historical, physical, and laboratory data must be the voice of the patient. In this case, some clues are not detectable above a whisper, some are dissonant, and others are loud but not completely clear. A history of consanguinity should always heighten awareness of an autosomal recessive condition. Homocystinuriais typically accompanied by characteristic physical findings, including lens dislocation, Marfanoid body habitus, and osteoporosis, but it is often not possible to appreciate these in an infant. The characteristic thromboembolic events seen in homocystinuria are also rare early in life. Macrocytic anemia is consistent with cobalamin or folate deficiency, but microcytosis suggests another dietary shortcoming, perhaps involving iron. Finally, concurrent increases in circulating methylmalonic acid and homocysteine are not limited to dietary deficiency. This biochemical pattern occurs in genetic defects involving cobalamin transport, cellular uptake, and lysosomal and cytoplasmic processing that prevent formation of functional adenosyl- and methylcobalamin. In light ofnormal methionine concentrations, aggregate data in this patient pointed to dietary insufficiency before treatment.

This case reminds us that biochemical patterns are dynamic and context-dependent. Yogi Berra admonished us to remember that "it's not over until it's over." Astute follow-up care, therefore, must include accurate assessment of therapeutic response and a willingness to reconsider a diagnosis when new evidence indicates. Missed opportunities to refine a diagnosis in kids have potentially serious, lifelong consequences.

Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.

Authors' Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest:

Employment or Leadership: D.J. Dietzen, The Journal of Applied Laboratory Medicine, AACC.

Consultant or Advisory Role: None declared.

Stock Ownership: None declared.

Honoraria: None declared.

Research Funding: None declared.

Expert Testimony: None declared.

Patents: None declared.

Received February 20, 2018; accepted February 23, 2018.

DOI: 10.1373/clinchem.2018.288142

Dennis J. Dietzen *

Departments of Pathology & Immunology, and Pediatrics, Washington University School of Medicine, St. Louis, MO.

* Address correspondence to the author at: 660 S. Euclid Ave Box 8118, St. Louis, MO 63110. Fax 314-454-2274;
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Title Annotation:Clinical Case Study
Author:Dietzen, Dennis J.
Publication:Clinical Chemistry
Date:Nov 1, 2018
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