A 6-month-old male baby comes with vomiting, lethargy and severe jaundice when weaning was staed with fruit juice. Which of the following enzymes is defective?
**Core Concept**
Aldolase B is a key enzyme involved in the breakdown of fructose in the liver. It catalyzes the conversion of fructose-1-phosphate to glyceraldehyde and dihydroxyacetone phosphate in the fructose metabolism pathway. Deficiency of this enzyme leads to a condition known as hereditary fructose intolerance (HFI), characterized by severe metabolic disturbances upon fructose ingestion.
**Why the Correct Answer is Right**
The correct answer is C. Aldolase B because it is the enzyme responsible for the breakdown of fructose in the liver. In HFI, the deficiency of aldolase B leads to the accumulation of toxic metabolites, causing symptoms such as vomiting, lethargy, and jaundice. The enzyme's deficiency is particularly relevant when the child is weaned onto a diet that includes fruit juice, which is high in fructose.
**Why Each Wrong Option is Incorrect**
**Option A:** Fructokinase is another enzyme involved in fructose metabolism, but it catalyzes the conversion of fructose to fructose-1-phosphate, which is actually the substrate for aldolase B. Deficiency of fructokinase leads to a different condition known as essential fructosuria.
**Option B:** Aldolase A is a different isoform of the aldolase enzyme, primarily involved in the breakdown of glucose-6-phosphate in muscle and brain tissues. It is not directly related to fructose metabolism.
**Option D:** Sucrase is an enzyme involved in the breakdown of sucrose into glucose and fructose, but it is not directly involved in the metabolism of fructose itself.
**Clinical Pearl / High-Yield Fact**
Hereditary fructose intolerance is a rare but potentially life-threatening condition that can be managed with a strict fructose-free diet. It is essential for clinicians to recognize the symptoms and promptly diagnose the condition to prevent long-term liver damage and other complications.
**✓ Correct Answer: C. Aldolase B**