A 6-month-old child undergoes a university medical center metabolic evaluation because of persistent hypoglycemia, hyperammonemia, skeletal muscle weakness, and myoglobinuria. After detailed studies, defective carnitine biosynthesis is identified. This molecule is necessary for which of the following biochemical functions?

A. Carboxylation of glutamic acid
B. Decarboxylation of alpha-ketoacids
C. Synthesis of 1,25-dihydroxycholecalciferol
D. Synthesis of rhodopsin ii photoreceptor pigments of the eye
E. Transport of long chain fatty acids into mitochondria

My guess is B. Decarboxylation of alpha-ketoacids . This is the carnitine cycle.

The correct answer is E. Carnitine is a methylated derivative of lysine that is used in the enzyme carnitine palmitoyltransferase. This enzyme catalyzes the movement of a long chain fatty acyl group from CoA to carnitine, and then the fatty acid attached to carnitine is carried across the inner mitochondrial membrane and into the inner mitochondrial space. Within the mitochondrion, fatty acyl CoA is regenerated and undergoes beta oxidation. Carnitine is obtained, to some degree, from diet and to a greater degree by endogenous synthesis. Carnitine deficiency can result from either increased demand (e.g., ketosis) coupled with inadequate diet, or from defects in the carnitine metabolic pathways. In addition to features noted in the question stem, carnitine deficiency may cause fatty liver, lipid storage myopathy, and confusion.
Carboxylation of glutamic acid (choice A) occurs in the synthesis of blood clotting factors and requires vitamin K.

Decarboxylation of alpha-ketoacids (choice B) requires thiamine.

Synthesis of 1,25-dihydroxycholecalciferol (choice C) requires vitamin D.

Synthesis of rhodopsin (choice D) requires vitamin A.