**Question:** Blood group antigen NAG (N-acetylgalactosamine) transportase present but galactosyltranferase is absent in

A. A
B. B
C. C
D. D

**Correct Answer:** .

**Core Concept:** Blood group antigens are carbohydrate molecules that are expressed on the surface of red blood cells (RBCs) and serve as specific markers for the identification of individuals and their compatibility for blood transfusion. Antigen NAG (N-acetylgalactosamine) is present in some blood groups, while galactosyltranferase plays a crucial role in the synthesis of galactose-containing antigens.

**Why the Correct Answer is Right:** Blood group antigens are classified into two categories: A and B antigens. They are synthesized through the actions of specific enzymes, such as galactosyltranferase (which adds galactose to the antigens) and NAG transporter (which adds N-acetylgalactosamine). In individuals with blood group C, galactosyltranferase is absent, leading to the absence of galactose-containing antigens. However, NAG transporter is present, which explains the presence of N-acetylgalactosamine-containing antigens in these individuals.

**Why Each Wrong Option is Incorrect:**

A. Individuals with blood group A possess galactosyltranferase but lack NAG transporter, resulting in the absence of N-acetylgalactosamine-containing antigens.

B. Blood group B individuals have both galactosyltranferase and NAG transporter, explaining the presence of galactose- and N-acetylgalactosamine-containing antigens.

C. Blood group O individuals possess both galactosyltranferase and NAG transporter, leading to the presence of galactose- and N-acetylgalactosamine-containing antigens.

D. Individuals with blood group AB possess both enzymes (galactosyltranferase and NAG transporter) and therefore have antigens from both groups A and B.

**Core Concept:** Blood group antigens are synthesized by specific enzymes, and their absence or presence is determined by the activity of these enzymes.

**Why Each Wrong Option is Incorrect:**

A. Blood group A individuals lack galactosyltranferase, which is responsible for the synthesis of galactose-containing antigens. This absence of the enzyme results in the absence of antigens in blood group A individuals.

B. Blood group B individuals possess both galactosyltranferase and NAG transporter, leading to the presence of galactose- and N-acetylgalactosamine-containing antigens in blood group B individuals.

C. Blood group O individuals possess both galactosyltranferase and NAG transporter, resulting in the presence of galactose- and N-acetylgalactosamine-containing antigens in blood group O individuals.

D. Blood group AB individuals have galactosyltranferase and NAG transporter, resulting in antigens from both groups A and B in these