NEET-PG 2012 Practice Questions

Q: The primary defect which leads to sickle cell anemia is:

Replacement of glutamate by valine in β-chain of HbA. ***Replacement of glutamate by valine in β-chain of HbA*** - The primary defect in sickle cell anemia is a **point mutation** that leads to the replacement of **glutamic acid** with **valine** in the **sixth position** of the β-globin chain [1]. - This mutation causes the hemoglobin (HbS) to polymerize under low oxygen conditions, resulting in the characteristic **sickle-shaped red blood cells** [1]. *A nonsence mutation in the I3-chain of HbA* - A nonsense mutation leads to a **premature stop codon**, which is not the mechanism behind sickle cell anemia. - This mutation does not involve the β-globin chain, which is critical in this specific disorder. *Substitution of valine by glutamate in the a-chain of HbA* - This statement is incorrect as sickle cell anemia specifically involves the **β-chain**, not the **α-chain**. - Substituting valine with glutamate does not cause sickling but rather the opposite of the actual defect observed in this condition. *An abnormality in porphyrin part of hemoglobin* - Sickle cell anemia does not arise from **porphyrin metabolism issues**, which are related to conditions like **porphyrias**. - The primary defect is a change in the amino acid sequence, not in the porphyrin structure of hemoglobin. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 598-599.

Q: What is the rate-limiting enzyme in heme synthesis?

ALA synthase. ***ALA synthase*** - **Aminolevulinate synthase** (ALA synthase) is the first and **rate-limiting enzyme** in the heme synthesis pathway. - Its activity is tightly regulated, and its overexpression or deficiency can lead to disorders like **acute intermittent porphyria**. *Hmg coa reductase* - **HMG-CoA reductase** is the **rate-limiting enzyme** in the **cholesterol biosynthesis pathway**, not heme synthesis. - It is the target enzyme for statin medications, which lower cholesterol levels. *ALA dehydratase* - **ALA dehydratase** (also known as porphobilinogen synthase) is the second enzyme in the heme synthesis pathway, responsible for converting two molecules of **ALA to porphobilinogen**. - While critical, it is not the rate-limiting step; inhibition of this enzyme can lead to **lead poisoning**. *Uroporphyrinogen 1 synthase* - **Uroporphyrinogen I synthase** (also called hydroxymethylbilane synthase or porphobilinogen deaminase) catalyzes the formation of **hydroxymethylbilane** from four molecules of **porphobilinogen**. - A deficiency in this enzyme is associated with **acute intermittent porphyria**, but it is not the rate-limiting enzyme of the overall pathway.

Q: Which enzyme is primarily responsible for the fat metabolism in adipose tissue?

Hormone-sensitive lipase. ***Hormone-sensitive lipase*** - This enzyme is crucial for the **mobilization of stored triglycerides** in adipose tissue by hydrolyzing them into fatty acids and glycerol. - Its activity is stimulated by hormones like **epinephrine** and **norepinephrine** and inhibited by insulin, reflecting its role in regulating fat release during energy demand. *Lipoprotein lipase* - This enzyme is primarily located on the **endothelial surface of capillaries** in various tissues, including adipose tissue, muscle, and heart. - Its main role is to clear **triglyceride-rich lipoproteins** like chylomicrons and VLDL from the bloodstream, facilitating the uptake of fatty acids into cells for storage or energy, rather than direct fat metabolism within the adipose cell. *Acid lipase* - **Lysosomal acid lipase** functions within lysosomes to break down cholesterol esters and triglycerides that are taken up by cells. - Its primary role is in the degradation of lipids within the **lysosomal compartments**, not in the primary process of fat mobilization from adipose tissue stores. *Acid maltase* - Also known as **alpha-glucosidase**, this enzyme is a lysosomal enzyme responsible for breaking down glycogen into glucose. - Its function is related to **glycogen metabolism** and has no direct role in fat metabolism in adipose tissue.

Q: Enzyme deficient in Type I Hyperlipidemia?

Lipoprotein lipase. ***Lipoprotein lipase*** - **Type I hyperlipidemia**, also known as **familial hyperchylomicronemia**, is characterized by a deficiency in **lipoprotein lipase (LPL)**. - LPL is crucial for hydrolyzing triglycerides in **chylomicrons** and **VLDLs** into fatty acids and glycerol, allowing their uptake by tissues. *HMG CoA reductase* - This enzyme is involved in the **rate-limiting step of cholesterol synthesis** in the liver. - While it plays a role in lipid metabolism, its deficiency is not characteristic of **Type I hyperlipidemia**. *Peroxidase* - **Peroxidase** is an enzyme involved in various oxidative reactions, including the breakdown of **hydrogen peroxide**. - It is not directly involved in the metabolism of **chylomicrons** or **triglycerides**, and its deficiency is unrelated to hyperlipidemia. *Cholesterol acyl transferase* - This enzyme, often referring to **lecithin-cholesterol acyltransferase (LCAT)** or **acyl-CoA:cholesterol acyltransferase (ACAT)**, is involved in **cholesterol esterification**. - While important for cholesterol transport and storage, its deficiency is not the primary cause of **Type I hyperlipidemia**, which is marked by severe **chylomicronemia**.

Q: The thickness of the endometrium at the time of implantation is:

7 - 10 mm. ***7 - 10 mm*** - At the time of **implantation** (day 6-10 post-fertilization, around day 20-24 of the menstrual cycle), the endometrium is in the **mid-secretory phase** and measures **7-10 mm** in thickness. - This is the **optimal thickness** for successful embryo implantation, characterized by a receptive endometrium with **decidualization**, **spiral artery development**, and **glycogen-rich glandular secretions**. - Endometrial thickness <7 mm is associated with **poor implantation rates** and reduced pregnancy success. *3 - 4 mm* - An endometrial thickness of 3-4 mm is **too thin** for successful implantation. - This thickness is typically seen in the **early proliferative phase** (immediately after menstruation), not during the implantation window. - Thin endometrium (<7 mm) is associated with **poor receptivity** and lower pregnancy rates in both natural conception and assisted reproduction. *20 - 30 mm* - An endometrial thickness of 20-30 mm is **abnormally thick** and not conducive to normal implantation. - Such thickness may indicate **endometrial hyperplasia**, **polyps**, or other pathological conditions requiring investigation. *30 - 40 mm* - An endometrial thickness of 30-40 mm is **severely abnormal** and would likely prevent successful implantation. - This extreme thickness suggests significant pathology such as **endometrial hyperplasia** or **malignancy** and requires urgent evaluation.

Q: What is the term for the collapse of a lung?

Atelectasis. ***Atelectasis*** - **Atelectasis** is the technical term for the **collapse of a lung** or a part of a lung, leading to reduced or absent gas exchange. - It can be caused by **obstruction of the airway** (e.g., mucus plug, foreign body) or external compression on the lung [1]. *Emphysema* - **Emphysema** is a chronic lung disease characterized by the **destruction of the alveoli**, leading to permanent enlargement of airspaces [2]. - It results in reduced elastic recoil of the lungs and is a type of **COPD (chronic obstructive pulmonary disease)** [2]. *Bronchiectasis* - **Bronchiectasis** is a chronic condition where the airways (bronchi) become **abnormally widened** and scarred. - This widening leads to a buildup of mucus, making the lungs vulnerable to **recurrent infections**. *Bronchitis* - **Bronchitis** is an inflammation of the lining of the bronchial tubes, which carry air to and from your lungs. - It typically causes a **cough** with mucus production and can be acute or chronic.

Q: The primary site of vasopressin synthesis is

Supraoptic nucleus. ***Supraoptic nucleus*** - The **supraoptic nucleus** of the hypothalamus is the **primary site** for the synthesis of **vasopressin** (also known as antidiuretic hormone or ADH). - Approximately **80% of vasopressin** is produced by the neurosecretory cells in this nucleus. - The synthesized vasopressin is transported down axons to the posterior pituitary for storage and release. *Preoptic nucleus* - The **preoptic nucleus** is involved in various homeostatic functions, including **thermoregulation** and **sleep regulation**, but not the synthesis of vasopressin. - While it has neuronal connections to the hypothalamus, it does not produce ADH. *Paraventricular nucleus* - The **paraventricular nucleus** also synthesizes **both vasopressin and oxytocin**, accounting for approximately **20% of vasopressin production**. - While it does produce vasopressin, the **supraoptic nucleus remains the primary site**, making it the correct answer to this question. - The PVN also plays important roles in stress response and various autonomic functions. *Posterior pituitary* - The **posterior pituitary** (neurohypophysis) is responsible for the **storage and release** of vasopressin and oxytocin, not their synthesis. - These hormones are produced in the hypothalamic nuclei (supraoptic and paraventricular) and then transported down axonal tracts to the posterior pituitary.

Q: Which of the following components are included in microcirculation?

Capillaries, venules, and arterioles. ***Capillaries, venules, and arterioles*** - **Microcirculation** is the portion of the **circulatory system** that includes the **smallest blood vessels**, specifically the **arterioles**, **capillaries**, and **venules**. - These vessels are crucial for the **delivery of oxygen** and **nutrients** to tissues and the removal of waste products. *Capillaries* - While **capillaries** are a vital part of **microcirculation** and the primary site of nutrient and waste exchange, they alone do not encompass the entire microcirculatory unit. - The microcirculation also includes the vessels that feed into and drain from the capillaries: the **arterioles** and **venules**. *Aorta* - The **aorta** is the **largest artery** in the body, part of the **macrocirculation**, which distributes blood from the heart to the systemic circulation. - It is not considered part of the **microcirculation** due to its large size and primary function as a high-pressure conduit rather than a site of exchange. *Arteries and veins* - **Arteries** and **veins** are primarily components of the **macrocirculation**, responsible for transporting blood to and from the systemic and pulmonary circuits. - While arterioles and venules (small arteries and veins) are part of the microcirculation, the broader terms "arteries" and "veins" typically refer to the larger vessels and do not exclusively define the microcirculatory network.

Question 1

The primary defect which leads to sickle cell anemia is:

Accepted Answer

Replacement of glutamate by valine in β-chain of HbA

Answer

An abnormality in porphyrin part of hemoglobin

Answer

A nonsense mutation in the β-chain of HbA

Answer

Substitution of valine by glutamate in the α-chain of HbA

Question 2

What is the rate-limiting enzyme in heme synthesis?

Accepted Answer

ALA synthase

Answer

HMG CoA reductase

Answer

ALA dehydratase

Answer

Uroporphyrinogen 1 synthase

Question 3

Which enzyme is primarily responsible for the fat metabolism in adipose tissue?

Accepted Answer

Hormone-sensitive lipase

Answer

Lipoprotein lipase

Answer

Acid lipase

Answer

Acid maltase

Question 4

What is the function of DNA ligase?

Accepted Answer

Joins the Okazaki fragments on the lagging strand by sealing the nicks between them.

Answer

Unwinding (denaturation) of dsDNA to provide an ssDNA template

Answer

Initiation of DNA synthesis and elongation

Answer

Initiates synthesis of RNA primers

Question 5

Which of the following represents the most significant regulatory control point among these TCA cycle reactions?

Accepted Answer

Isocitrate to Alpha-ketoglutarate (Isocitrate dehydrogenase)

Answer

Succinyl-CoA to Succinate (Succinyl-CoA synthetase)

Answer

Acetyl-CoA + Oxaloacetate to Citrate (Citrate synthase)

Answer

Alpha-ketoglutarate to Succinyl-CoA (Alpha-ketoglutarate dehydrogenase complex)

Question 6

Enzyme deficient in Type I Hyperlipidemia?

Accepted Answer

Lipoprotein lipase

Answer

HMG CoA reductase

Answer

Peroxidase

Answer

Cholesterol acyl transferase

Question 7

The thickness of the endometrium at the time of implantation is:

Accepted Answer

7 - 10 mm

Answer

20 - 30 mm

Answer

30 - 40 mm

Answer

3 - 4 mm

Question 8

What is the term for the collapse of a lung?

Accepted Answer

Atelectasis

Answer

Emphysema

Answer

Bronchitis

Answer

Bronchiectasis

Question 9

The primary site of vasopressin synthesis is

Accepted Answer

Supraoptic nucleus

Answer

Preoptic nucleus

Answer

Paraventricular nucleus

Answer

Posterior pituitary

Question 10

Which of the following components are included in microcirculation?

Accepted Answer

Capillaries, venules, and arterioles

Answer

Capillaries

Answer

Aorta

Answer

Arteries and veins

NEET-PG 2012

Biochemistry

Obstetrics and Gynecology

Pathology

Physiology