NEET-PG 2017 - Biochemistry PYQs Practice Questions

Q: The following lesion is seen in all of the following conditions except: (Recent NEET Pattern 2016-17)

Porphyria. ***Correct: Porphyria*** - **Porphyria** causes **photosensitive skin lesions** but does NOT cause pellagra or the **Casal's necklace** pattern shown in the image. - Porphyria results from defects in **heme synthesis**, leading to vesicles, bullae, and hyperpigmentation, particularly on sun-exposed areas. - The image depicts **Pellagra**, characterized by **photosensitive dermatosis** in a **Casal's necklace** distribution, caused by **niacin (vitamin B3) deficiency**. *Incorrect: Carcinoid syndrome* - **Carcinoid syndrome** can cause **pellagra** due to **tryptophan diversion** for excessive serotonin synthesis. - This leads to **niacin deficiency** and can present with the characteristic skin lesions shown in the image. - Classic features include flushing, diarrhea, and cardiac involvement. *Incorrect: Hartnup disease* - **Hartnup disease** is an **autosomal recessive disorder** with impaired absorption of neutral amino acids, including **tryptophan**. - Reduced tryptophan absorption leads to **secondary niacin deficiency**, causing pellagra-like symptoms. - Patients present with photosensitive dermatitis, cerebellar ataxia, and psychiatric manifestations. *Incorrect: Alcoholism* - **Chronic alcoholism** is a common cause of **pellagra** due to poor dietary intake, malabsorption, and impaired nutrient metabolism. - Alcoholics are at high risk for **niacin deficiency**, presenting with the **3 Ds**: dermatitis, diarrhea, and dementia. - The photosensitive rash in a Casal's necklace distribution is characteristic. [Practice more Biochemistry PYQs on OnCourse]

Q: A patient reports a change in colour of urine on air exposure. All are true about the condition shown below except:

Urine Benedict's test is negative. ***Urine Benedict's test is negative*** - This is FALSE - Benedict's test is actually **POSITIVE** in alkaptonuria because **homogentisic acid** is a reducing agent. - Homogentisic acid readily **reduces Benedict's reagent**, giving a positive test result in alkaptonuria patients. *Blackening of urine is accelerated on exposure to sunlight* - This is TRUE - **UV light** and sunlight accelerate the **oxidation of homogentisic acid** in urine. - The characteristic **dark discoloration** occurs more rapidly when exposed to light and air. *Alkaptone bodies are deposited in intervertebral disc* - This is TRUE - **Homogentisic acid (alkaptone bodies)** polymerizes to form **ochronotic pigment** deposits. - These deposits accumulate in **cartilage** including intervertebral discs, causing degenerative changes and spondylosis. *The condition is caused by deficiency of homogentisate 1,2-dioxygenase* - This is TRUE - Alkaptonuria is caused by deficiency of **homogentisate 1,2-dioxygenase** enzyme in the **tyrosine metabolic pathway**. - This enzyme deficiency leads to accumulation of **homogentisic acid** in blood and urine, causing the characteristic symptoms. [Practice more Biochemistry PYQs on OnCourse]

Q: Name the product marked as X in the image shown below:

Indolequinone. ***Indolequinone*** - The image depicts the **melanin biosynthesis pathway** starting from **tyrosine**. - Tyrosine is converted to **L-DOPA** by tyrosinase, which is then oxidized to **dopaquinone** (also called DOPA-quinone). - Dopaquinone undergoes intramolecular cyclization to form **leucodopachrome**, which is subsequently oxidized to **dopachrome**. - Dopachrome is then converted through a series of steps to **5,6-dihydroxyindole**, which is finally oxidized to **indole-5,6-quinone** (indolequinone). - **Indolequinone** is a key intermediate that polymerizes to form **melanin**, the pigment responsible for coloration in skin, hair, and eyes. - Based on the pathway shown, X represents indolequinone, an oxidized indole derivative in melanin synthesis. *Tetrabenazine* - **Tetrabenazine** is a pharmaceutical drug that inhibits **vesicular monoamine transporter 2 (VMAT2)**. - It is used therapeutically to treat hyperkinetic movement disorders such as **Huntington's disease** and tardive dyskinesia. - It is not a natural metabolic intermediate in the tyrosine-to-melanin biosynthetic pathway. *Homovanillic acid* - **Homovanillic acid (HVA)** is a major end metabolite of **dopamine** in the catecholamine degradation pathway. - It is formed by the sequential actions of **monoamine oxidase (MAO)** and **catechol-O-methyltransferase (COMT)** on dopamine. - This represents a completely different branch of tyrosine metabolism (catecholamine pathway), not the melanin synthesis pathway. *Kynurenine* - **Kynurenine** is an intermediate metabolite in the **kynurenine pathway**, which is the major route of **tryptophan** degradation. - The kynurenine pathway leads to the formation of NAD+ and various neuroactive metabolites. - This pathway is completely separate from tyrosine metabolism and is unrelated to the melanin synthesis pathway shown in the image. [Practice more Biochemistry PYQs on OnCourse]

Q: All are correct about the amino acid marked as X, EXCEPT:

Ketogenic. ***Correct Answer: Ketogenic*** ✓ - Histidine is **purely glucogenic**, NOT ketogenic - Its carbon skeleton is converted to α-ketoglutarate (a TCA cycle intermediate), which can be used for gluconeogenesis - It does NOT produce acetyl-CoA or acetoacetate (ketone body precursors) - **This statement is FALSE, making it the correct answer for this EXCEPT question** *Incorrect: Maximum buffering action* - This statement is TRUE (so not the answer) - Histidine contains an **imidazole ring** with pKa ~6.0, close to physiological pH - Provides crucial **buffering capacity** in blood and tissues - Most effective amino acid buffer at physiological pH *Incorrect: Contributes to one carbon pool* - This statement is TRUE (so not the answer) - Histidine breakdown produces **Formimino glutamic acid (FIGLU)** - FIGLU donates its formimino group to **Tetrahydrofolate (THF)** - Contributes to the **one-carbon pool** essential for biosynthetic pathways *Incorrect: Excess amount leads to mental retardation* - This statement is TRUE (so not the answer) - **Histidinemia** results from histidase enzyme deficiency - Elevated histidine levels can be associated with **developmental delays and speech defects** in some cases - Though many affected individuals remain asymptomatic [Practice more Biochemistry PYQs on OnCourse]

Q: Name the plot dealing with kinetics of enzyme inhibition?

Dixon plot. ***Dixon plot*** - The **Dixon plot** is specifically used to determine the type of enzyme inhibition and to calculate the **inhibition constant (Ki)**. - It plots the reciprocal reaction velocity (1/V) against the **inhibitor concentration ([I])** at different substrate concentrations. *Donnan-Gibbs equation plot* - The **Donnan-Gibbs equation** describes the distribution of charged particles across a semi-permeable membrane at equilibrium. - It is irrelevant to enzyme kinetics or inhibition. *Lineweaver-Burk plot* - While the Lineweaver-Burk plot is used in enzyme kinetics to determine **Km** and **Vmax** and analyze inhibition, it plots **1/V against 1/[S]**. - The image provided, showing 1/V against [I], is characteristic of a Dixon plot, not a Lineweaver-Burk plot which plots 1/V against 1/[S]. *Hanes-Woolf plot* - The **Hanes-Woolf plot** is another linearization of the Michaelis-Menten equation, plotting **[S]/V against [S]**. - Like the Lineweaver-Burk plot, it is used for analyzing general enzyme kinetics but not specifically for determining Ki from varying inhibitor concentrations in the manner shown. [Practice more Biochemistry PYQs on OnCourse]

Question 1

The following lesion is seen in all of the following conditions except: (Recent NEET Pattern 2016-17)

Accepted Answer

Porphyria

Answer

Carcinoid syndrome

Answer

Hartnup disease

Answer

Alcoholism

Question 2

A patient reports a change in colour of urine on air exposure. All are true about the condition shown below except:

Accepted Answer

Urine Benedict's test is negative

Answer

Blackening of urine is accelerated on exposure to sunlight

Answer

Alkaptone bodies are deposited in intervertebral disc

Answer

The condition is caused by deficiency of homogentisate 1,2-dioxygenase

Question 3

Name the product marked as X in the image shown below:

Accepted Answer

Indolequinone

Answer

Tetrabenazine

Answer

Homovanillic acid

Answer

Kynurenine

Question 4

All are correct about the amino acid marked as X, EXCEPT:

Accepted Answer

Ketogenic

Answer

Maximum buffering action

Answer

Contributes to one carbon pool

Answer

Excess amount leads to mental retardation

Question 5

Name the plot dealing with kinetics of enzyme inhibition?

Accepted Answer

Dixon plot

Answer

Donnan-Gibbs equation plot

Answer

Lineweaver-Burk plot

Answer

Hanes-Woolf plot

Question 6

Which enzyme marked as $X$ is missing in synthesis of vitamin $D_{3}$ ?

Accepted Answer

17-alpha-hydroxylase

Answer

3-beta-hydroxylase

Answer

21-hydroxylase

Answer

24-hydroxylase

Question 7

The following reaction occurs in which part of kidney?

Accepted Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Loop of Henle

Answer

Collecting duct

Question 8

Name the antigen marked as X determining blood group A.

Accepted Answer

N-Acetyl-Galactosamine

Answer

N-Acetyl-Glucosamine

Answer

Dermatan sulphate

Answer

Keratan sulfate

NEET-PG 2017 — Biochemistry