Galactosaemia commonly is due to deficiency of-
**Core Concept**
Galactosaemia is a rare genetic disorder characterized by the inability to metabolize galactose, a sugar found in milk and other dairy products. This condition arises due to defects in enzymes involved in the Leloir pathway, a crucial metabolic pathway for galactose metabolism.
**Why the Correct Answer is Right**
Galactosaemia is primarily caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase (GALT), which catalyzes the conversion of galactose-1-phosphate to UDP-galactose in the Leloir pathway. This defect leads to the accumulation of toxic levels of galactose-1-phosphate, causing cellular damage and organ dysfunction.
**Why Each Wrong Option is Incorrect**
* **Option A:** Galactokinase (GALK) deficiency is another cause of galactosaemia, but it is less common than GALT deficiency.
* **Option B:** UDP-galactose 4'-epimerase (GALE) deficiency is associated with a rare form of galactosaemia, but it is not the most common cause.
* **Option D:** No enzyme deficiency is listed, making it an incorrect option.
**Clinical Pearl / High-Yield Fact**
Galactosaemia can be diagnosed through newborn screening, and prompt treatment with a galactose-free diet can prevent long-term complications, such as liver dysfunction, cataracts, and developmental delays.
**Correct Answer:** C.