NEET-PG 2021 — Biochemistry
8 Previous Year Questions with Answers & Explanations
A patient on a maize diet presented with diarrhea, dementia and dermatitis. Which vitamin deficiency is responsible for these features
Which amino acid needs to be supplemented through diet in patient with cystathionine beta synthase deficiency
A patient had dinner at 8 PM at night and does his blood sugar test at 7 AM in the morning. What is the major source of glucose at this time?
A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient
A patient with tendon xanthomas, Increased LDL and cholesterol. What is the most probable diagnosis?
A farmer complains of a skin rash that worsens with sun exposure and redness of the tongue. Maize is the staple in his diet. Which vitamin deficiency is likely responsible for these symptoms?
A young boy with a thin build and long fingers presents with diminished vision. On examination, subluxation of the lens is observed, and cystathionine synthase deficiency is detected. Which amino acid should the patient be supplemented with?
A software engineer presents to the OPD with 'complaints of easy fatigability. He reports sitting in front of a computer for 12-14 hours a day consuming junk food, and eating few fruits and vegetables. CBC results show hemoglobin (Hb) concentration of $7 \mathrm{gm} \%$ and MCV of 120 fL . What is the most likely cause of anemia?
NEET-PG 2021 - Biochemistry NEET-PG Practice Questions and MCQs
Question 1: A patient on a maize diet presented with diarrhea, dementia and dermatitis. Which vitamin deficiency is responsible for these features
- A. Niacin (Correct Answer)
- B. Riboflavin
- C. Thiamine
- D. Pyridoxine
- E. Cobalamin
Explanation: ***Niacin*** - The classic presentation of **pellagra**, caused by a deficiency of **niacin (Vitamin B3)**, is characterized by the "**3 Ds**": **dermatitis**, **diarrhea**, and **dementia**. In severe cases, a fourth 'D' for death can also occur. - A **maize (corn)** staple diet is a known risk factor for niacin deficiency because maize contains niacin in a bound, non-bioavailable form (niacytin) and is low in tryptophan, a precursor to niacin. *Riboflavin* - **Riboflavin (Vitamin B2)** deficiency leads to **ariboflavinosis**, which can cause **cheilosis**, **angular stomatitis**, **glossitis**, and **seborrheic dermatitis**, but not the constellation of diarrhea, dementia, and dermatitis seen in pellagra. - It does not typically manifest with neurological or gastrointestinal symptoms as severe as those described in the question. *Thiamine* - **Thiamine (Vitamin B1)** deficiency causes **beriberi**, characterized by **neurological (dry beriberi)** or **cardiovascular (wet beriberi)** symptoms. - It can lead to **Wernicke-Korsakoff syndrome** in severe cases, which includes neurological deficits, but not the specific "3 Ds" of pellagra. *Pyridoxine* - **Pyridoxine (Vitamin B6)** deficiency can cause **neurological symptoms** such as **peripheral neuropathy**, **seizures**, and **depression**, as well as **dermatitis** and **glossitis**. - It does not present with the characteristic triad of dermatitis, diarrhea, and dementia seen in pellagra. *Cobalamin* - **Cobalamin (Vitamin B12)** deficiency causes **megaloblastic anemia** and **neurological symptoms** including **subacute combined degeneration** of the spinal cord, **peripheral neuropathy**, and **cognitive changes**. - While it can cause neurological symptoms, it does not present with the classic dermatitis and diarrhea combination seen in pellagra.
Question 2: Which amino acid needs to be supplemented through diet in patient with cystathionine beta synthase deficiency
- A. Tryptophan
- B. Serine
- C. Methionine
- D. Cysteine (Correct Answer)
- E. Tyrosine
Explanation: ***Cysteine*** - In **cystathionine beta synthase (CBS) deficiency**, the conversion of **homocysteine** to **cystathionine** (and subsequently to cysteine) is impaired. - This makes **cysteine** an **essential amino acid** for these patients, requiring dietary supplementation. *Tryptophan* - **Tryptophan** is an **essential amino acid** and a precursor for **serotonin** and **niacin**, but its metabolism is not directly affected by CBS deficiency. - Its supplementation is not specifically indicated for this condition. *Serine* - **Serine** is a **non-essential amino acid** that provides the **carbon skeleton** for the synthesis of cysteine from homocysteine in the presence of CBS. - While important in the pathway, CBS deficiency specifically disrupts the downstream conversion of homocysteine, making **cysteine** the deficient product, not serine. *Methionine* - **Methionine** is an **essential amino acid** that is a precursor to **homocysteine**; in CBS deficiency, homocysteine levels are already elevated due to impaired conversion to cystathionine. - Restricting methionine intake is typically recommended in CBS deficiency to reduce homocysteine accumulation, not supplementing it. *Tyrosine* - **Tyrosine** is a **non-essential amino acid** derived from **phenylalanine** and serves as a precursor for catecholamines and thyroid hormones. - Its metabolism is not affected by CBS deficiency, and supplementation is not indicated for this condition.
Question 3: A patient had dinner at 8 PM at night and does his blood sugar test at 7 AM in the morning. What is the major source of glucose at this time?
- A. Liver Glycogen (Correct Answer)
- B. Muscle Glycogen
- C. Gluconeogenesis
- D. Dietary Carbohydrate
- E. Ketone bodies
Explanation: ***Liver Glycogen*** - After an overnight fast (approximately 11 hours in this scenario), the primary mechanism for maintaining blood glucose levels is the breakdown of **liver glycogen** stores. - The liver is crucial for glucose homeostasis as it can release glucose directly into the bloodstream, a function muscle glycogen cannot perform. *Muscle Glycogen* - **Muscle glycogen** serves as an energy reserve primarily for the muscle itself and cannot be directly released into the bloodstream to maintain blood glucose levels. - It is utilized for physical activity and local energy demands within muscle cells. *Gluconeogenesis* - **Gluconeogenesis**, the synthesis of glucose from non-carbohydrate precursors, becomes increasingly important for glucose production after prolonged fasting (typically *after* liver glycogen stores are depleted). - While it contributes during an overnight fast, **liver glycogenolysis** is the dominant source initially. *Dietary Carbohydrate* - **Dietary carbohydrates** from the previous dinner (8 PM) would have been absorbed and utilized or stored as glycogen much earlier than 7 AM the next morning. - By 7 AM, the direct impact of the previous night's meal on circulating glucose is negligible, having been processed hours before. *Ketone Bodies* - **Ketone bodies** are alternative fuel sources produced during prolonged fasting or starvation, but they are **not glucose**. - While they can be used by tissues (brain, heart, muscle) for energy during extended fasting, they do not contribute to blood glucose levels and are metabolically distinct from glucose.
Question 4: A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient
- A. Homogentisate dehydrogenase
- B. Homogentistae oxidase (Correct Answer)
- C. Tyrosine Transaminase
- D. Tryptophan Hydroxylase
- E. Phenylalanine Hydroxylase
Explanation: ***Homogentistae oxidase*** - The presentation of a child with **black urine** (alkaptonuria) in the absence of other symptoms is characteristic of a deficiency in **homogentisate oxidase**. - This enzyme is crucial in the catabolism of **tyrosine**, and its deficiency leads to the accumulation of **homogentisic acid**, which oxidizes upon exposure to air, turning urine black. *Homogentisate dehydrogenase* - This enzyme is not a recognized component of the **tyrosine degradation pathway** in humans. - The correct enzyme involved in the breakdown of **homogentisate** is an oxidase, not a dehydrogenase, in this context. *Tyrosine Transaminase* - A deficiency in **tyrosine transaminase** (tyrosinemia type II) would lead to elevated tyrosine levels and typically presents with symptoms affecting the eyes, skin, and intellectual disability, not primarily black urine. - This condition is characterized by **ocular findings** (corneal ulcers), **skin lesions**, and **neurological symptoms**. *Tryptophan Hydroxylase* - This enzyme is involved in the synthesis of **serotonin** and **melatonin** from tryptophan, a different amino acid pathway. - A deficiency or abnormality in **tryptophan hydroxylase** would not cause black urine but could lead to neurological or mood disorders. *Phenylalanine Hydroxylase* - A deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, which affects phenylalanine metabolism, not tyrosine metabolism directly. - PKU typically presents with **intellectual disability**, **musty odor**, **fair skin**, and **seizures** if untreated, not black urine.
Question 5: A patient with tendon xanthomas, Increased LDL and cholesterol. What is the most probable diagnosis?
- A. Type II Hyperlipoproteinemia (Correct Answer)
- B. Type III Hyperlipoproteinemia
- C. Abetalipoproteinemia
- D. Type I Hyperlipoproteinemia
- E. Type IV Hyperlipoproteinemia
Explanation: ***Type II Hyperlipoproteinemia*** - This type is characterized by significantly **elevated LDL and total cholesterol** due to a defect in LDL receptor function or APOB-100. - **Tendon xanthomas** are a classic physical finding in Type II hyperlipoproteinemia, specifically in familial hypercholesterolemia. *Type III Hyperlipoproteinemia* - This condition involves increased levels of **chylomicron remnants** and **VLDL remnants (IDL)**, leading to elevated cholesterol and triglycerides. - While xanthomas can occur (e.g., **palmar xanthomas**), tendon xanthomas are less typical, and the primary lipid abnormality isn't isolated LDL elevation. *Abetalipoproteinemia* - This is a rare autosomal recessive disorder resulting in the **absence of LDL, VLDL, and chylomicrons** in the blood. - Patients present with **fat malabsorption**, neurologic symptoms, and generally have very low or undetectable cholesterol and triglyceride levels, which is contrary to the clinical presentation. *Type I Hyperlipoproteinemia* - This disorder is characterized by a deficiency of **lipoprotein lipase (LPL)** or its cofactor, APO C-II, leading to extremely high levels of **chylomicrons** and **triglycerides**. - While eruptive xanthomas can be seen, **tendon xanthomas** are not a feature, and the primary abnormality is hypertriglyceridemia, not elevated LDL. *Type IV Hyperlipoproteinemia* - This condition is characterized by **elevated VLDL** and **triglycerides** with normal or slightly elevated LDL. - Xanthomas are generally not a feature, and the primary abnormality is hypertriglyceridemia rather than hypercholesterolemia with elevated LDL.
Question 6: A farmer complains of a skin rash that worsens with sun exposure and redness of the tongue. Maize is the staple in his diet. Which vitamin deficiency is likely responsible for these symptoms?
- A. Vitamin C deficiency
- B. Vitamin K deficiency
- C. Niacin deficiency (Correct Answer)
- D. Folic acid deficiency
- E. Riboflavin deficiency
Explanation: ***Niacin deficiency*** - The symptoms described (dermatitis that worsens with sun exposure, glossitis, and a diet high in maize) are characteristic of **pellagra**, which is caused by a **niacin (Vitamin B3) deficiency**. - **Maize** is a poor source of niacin, and the niacin it contains is largely bound and unavailable, leading to higher rates of deficiency in populations relying on it as a staple. - The classic **3 D's of pellagra** are: **Dermatitis** (photosensitive), **Diarrhea**, and **Dementia**. *Riboflavin deficiency* - **Riboflavin (Vitamin B2) deficiency** causes **angular stomatitis**, **cheilosis**, and **glossitis**, but the dermatitis is typically **seborrheic** (not photosensitive). - The **photosensitive** nature of the rash in this case is the key differentiating feature pointing to niacin deficiency. *Vitamin C deficiency* - **Vitamin C deficiency** leads to **scurvy**, characterized by **gingival bleeding**, **poor wound healing**, and **petechiae**, none of which are mentioned here. - While it can manifest with fatigue, it does not typically cause a photosensitive rash or glossitis as described. *Vitamin K deficiency* - **Vitamin K deficiency** primarily affects **blood clotting**, leading to **bleeding disorders** and **easy bruising**. - It is not associated with skin rashes or glossitis. *Folic acid deficiency* - **Folic acid deficiency** causes **macrocytic anemia**, fatigue, and potentially **glossitis**, but it does not cause a photosensitive rash. - The distinctive combination of photosensitive dermatitis and glossitis points away from folic acid deficiency.
Question 7: A young boy with a thin build and long fingers presents with diminished vision. On examination, subluxation of the lens is observed, and cystathionine synthase deficiency is detected. Which amino acid should the patient be supplemented with?
- A. Tyrosine
- B. Serine
- C. Cysteine (Correct Answer)
- D. Methionine
- E. Glycine
Explanation: ***Cysteine*** - In **cystathionine synthase deficiency** (homocystinuria), the body cannot convert **homocysteine to cystathionine**, and subsequently to **cysteine**. - **Cysteine** therefore becomes an **essential amino acid** in these patients and must be supplemented. *Tyrosine* - **Tyrosine** is a non-essential amino acid synthesized from **phenylalanine**, and its deficiency is not directly related to cystathionine synthase deficiency. - It is not involved in the **methionine or homocysteine metabolic pathway** that is disrupted in homocystinuria. *Serine* - **Serine** is a substrate for the **cystathionine synthase enzyme**, which combines with **homocysteine** to form **cystathionine**. - Supplementation with serine alone would not bypass the enzyme deficiency or provide the essential product, **cysteine**. *Methionine* - **Methionine** is the precursor to **homocysteine**, and in cystathionine synthase deficiency, there's often an accumulation of methionine and homocysteine. - Therefore, **methionine restriction** is typically part of the treatment, not supplementation. *Glycine* - **Glycine** is involved in one-carbon metabolism but is not directly involved in the **transsulfuration pathway** affected by cystathionine synthase deficiency. - Glycine supplementation would not address the inability to synthesize **cysteine** from homocysteine.
Question 8: A software engineer presents to the OPD with 'complaints of easy fatigability. He reports sitting in front of a computer for 12-14 hours a day consuming junk food, and eating few fruits and vegetables. CBC results show hemoglobin (Hb) concentration of $7 \mathrm{gm} \%$ and MCV of 120 fL . What is the most likely cause of anemia?
- A. Cyanocobalamin deficiency
- B. Acute blood loss
- C. Sideroblastic anemia
- D. Folate deficiency (Correct Answer)
- E. Iron deficiency anemia
Explanation: ***Folate deficiency*** - A **macrocytic anemia** with an **MCV of 120 fL** is characteristic of folate deficiency, as folate is vital for **DNA synthesis** in red blood cell production. - The patient's diet of **junk food** and few fruits/vegetables suggests poor nutritional intake, as folate is abundant in leafy greens and fresh produce. *Cyanocobalamin deficiency* - While also causing **macrocytic anemia** with high MCV, cyanocobalamin (Vitamin B12) deficiency often presents with **neurological symptoms** (e.g., neuropathy, cognitive changes) which are not mentioned. - Dietary sources of B12 are primarily **animal products**, and while junk food is poor, a strict vegetarian/vegan diet is a stronger indicator of B12 deficiency. *Acute blood loss* - Acute blood loss typically causes **normocytic, normochromic anemia**, characterized by a normal MCV in the initial stages. - While severe blood loss can lead to fatigue, the **elevated MCV** of 120 fL makes this diagnosis unlikely unless there's a pre-existing macrocytic condition. *Sideroblastic anemia* - Sideroblastic anemia can be **microcytic, normocytic, or macrocytic**, but it is primarily characterized by the presence of **ring sideroblasts** in the bone marrow and iron overload. - It's often associated with **alcoholism, lead poisoning, or myelodysplastic syndromes**, and the typical features of the patient's diet and MCV do not point towards this condition. *Iron deficiency anemia* - Iron deficiency anemia presents with **microcytic, hypochromic anemia** with a **low MCV** (typically <80 fL), not macrocytic anemia. - While iron deficiency is the most common cause of anemia worldwide and can result from poor diet, the **elevated MCV of 120 fL** clearly excludes this diagnosis.