MBBS Biochemistry

Question 1

Which enzyme is deficient in Phenylketonuria (PKU)?

Accepted Answer

Phenylalanine hydroxylase

Answer

PKU is caused by deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine, leading to phenylalanine accumulation and intellectual disability.

Question 2

In which organelle does the citric acid (Krebs) cycle occur?

Accepted Answer

Mitochondrial matrix

Answer

The citric acid cycle occurs in the mitochondrial matrix, where acetyl-CoA is oxidized to CO2 with generation of NADH, FADH2, and GTP.

Question 3

Hemoglobin S (sickle cell) differs from hemoglobin A by a substitution of which amino acid?

Accepted Answer

Valine replaces glutamate at position 6 of the beta chain

Answer

Sickle cell disease results from a single point mutation causing valine to replace glutamic acid at position 6 of the beta-globin chain.

Question 4

Which vitamin serves as a coenzyme for pyruvate dehydrogenase?

Accepted Answer

Thiamine (B1)

Answer

Thiamine pyrophosphate (TPP) is a required cofactor for pyruvate dehydrogenase; its deficiency impairs aerobic metabolism, causing Wernicke's encephalopathy.

Question 5

DNA replication proceeds in which direction?

Accepted Answer

Bidirectional from multiple origins but always 5' to 3'

Answer

DNA replication is bidirectional from origins of replication, but each new strand is synthesized only in the 5' to 3' direction by DNA polymerase.

MBBS - Bachelor of Medicine and Bachelor of Surgery Practice Test

MBBS - Bachelor of Medicine and Bachelor of Surgery Practice Test

MBBS Biochemistry