Final answer:
A 4-month-old child exhibited extreme tiredness, irritable moods, poor appetite, and fasting hypoglycemia associated with vomiting and muscle weakness, the most likely molecular defect in this child is in the enzyme (B) Carnitine transporter.
Step-by-step explanation:
In the given scenario, the child is exhibiting symptoms such as extreme tiredness, poor appetite, and fasting hypoglycemia. The blood work showed elevated levels of free fatty acids but low levels of acylcarnitine. These symptoms and laboratory findings suggest a defect in the transport of fatty acids into the mitochondria, which is facilitated by the carnitine transporter.
The breakdown of fatty acids for energy occurs in the mitochondrial matrix, and the initial step involves the conversion of fatty acyl-CoA to fatty acyl carnitine. This conversion is facilitated by the carnitine transporter enzyme. However, if there is a defect in this transporter, fatty acids cannot be transported into the mitochondria for oxidation, leading to the symptoms observed in the child.
In contrast, the other enzymes listed in the options are not involved in the transport of fatty acids into the mitochondria. Medium-chain acyl-CoA dehydrogenase (Option A) is responsible for catalyzing the first step of fatty acid oxidation within the mitochondria, acetyl-CoA carboxylase (Option C) is involved in fatty acid synthesis, carnitine acyltransferase II (Option D) is responsible for converting fatty acyl carnitine back into fatty acyl-CoA within the mitochondria, and HMG-CoA synthase (Option E) is involved in the synthesis of ketone bodies.
Therefore the correct answer is (B) Carnitine transporter.