NEET-PG 2013 Practice Questions

Q: Which of the following enzymes is classified as a serine protease?

Trypsin. ***Trypsin*** - **Trypsin** is a digestive enzyme belonging to the **serine protease** family, characterized by a crucial **serine residue** in its active site. - It plays a vital role in protein digestion in the small intestine, cleaving peptide bonds on the carboxyl side of **lysine** or **arginine** residues. *Pepsin* - **Pepsin** is an aspartic protease, meaning it utilizes an **aspartate residue** in its active site for catalysis. - It primarily functions in the stomach, digesting proteins into smaller peptides in an **acidic environment**. *Carboxypeptidase* - **Carboxypeptidase** is a **metalloexopeptidase** that contains a zinc ion in its active site. - It removes amino acids one by one from the **carboxyl-terminal** end of polypeptide chains. *None of the options* - This option is incorrect because **trypsin** is indeed a well-known example of a serine protease.

Q: Rate limiting step in pyrimidine synthesis?

Carbamoyl phosphate synthase-II. ***Carbamoyl phosphate synthetase II (CPS-II)*** - **CPS-II** is the **committed and rate-limiting enzyme** in **de novo pyrimidine synthesis** in **mammals (including humans)** - It catalyzes the formation of **carbamoyl phosphate** from glutamine, CO₂, and 2 ATP in the **cytoplasm** - This is the **first committed step** and the main **regulatory checkpoint**, inhibited by UTP (feedback inhibition) and activated by PRPP and ATP - CPS-II is part of the **CAD complex** (carbamoyl phosphate synthetase, aspartate transcarbamoylase, dihydroorotase) in mammals *Aspartate transcarbamoylase (ATCase)* - ATCase catalyzes the **second step**: condensation of carbamoyl phosphate with aspartate to form carbamoyl aspartate - While ATCase is the **rate-limiting step in bacteria** (E. coli), in **mammals** it is part of the CAD complex and **not the primary regulatory step** - This option is incorrect for human/mammalian biochemistry tested in NEET PG *Dihydro-orotase* - The **third enzyme** in the pathway, converting carbamoyl aspartate to dihydroorotate - Part of the CAD complex in mammals but **not the rate-limiting step** *Dihydroorotate dehydrogenase* - Catalyzes the **fourth step**: oxidation of dihydroorotate to orotate - Located on the **outer surface of the inner mitochondrial membrane** (only mitochondrial enzyme in the pathway) - Important enzyme but **not rate-limiting**

Q: Glucagon stimulates

Gluconeogenesis. ***Gluconeogenesis*** - **Glucagon** is a hormone that primarily acts to raise **blood glucose levels** by stimulating the production of glucose from non-carbohydrate sources. - This process, **gluconeogenesis**, occurs mainly in the liver and is initiated by glucagon to counteract hypoglycemia. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from glucose and is primarily stimulated by insulin when blood glucose levels are high. - Glucagon's role is to *inhibit* glycogen synthesis and instead promote glycogen breakdown. *Fatty acid synthesis* - **Fatty acid synthesis** is an anabolic process that primarily occurs when there is an excess of energy and glucose, often stimulated by **insulin**. - Glucagon generally has an **inhibitory effect** on fatty acid synthesis, as its main goal is to mobilize energy stores, not create them. *Glycolysis* - **Glycolysis** is the breakdown of glucose to produce energy, and it is stimulated when glucose is abundant and energy is needed. - Glucagon primarily acts to *inhibit* glycolysis in the liver, thereby conserving glucose for use by other tissues and promoting its release into the bloodstream.

Q: Which of the following statements regarding collagen synthesis is incorrect?

Hydroxylation of proline occurs in Golgi apparatus. ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.

Q: What is the major fibril protein associated with Primary Amyloidosis?

Immunoglobulin Light Chain. ***AL*** - In Primary Amyloidosis, **AL amyloid** is derived from immunoglobulin light chains produced by **plasma cell dyscrasias** [1]. - This type of amyloidosis is commonly associated with conditions like **multiple myeloma** or monoclonal gammopathy [1]. *Transthyretin* - This protein is associated with **Familial Amyloid Polyneuropathy** and **Senile Systemic Amyloidosis**, not Primary Amyloidosis. - Transthyretin amyloidosis (ATTR) results from **mutations** or **aging**, contributing to different clinical presentations than AL. *AA* - AA amyloidosis is secondary and occurs due to **chronic inflammatory** conditions, such as rheumatoid arthritis or chronic infections. - It is not the main fibril protein in **Primary Amyloidosis**, which is specifically linked to **light chains**. *Procalcitonin* - Procalcitonin is a **biomarker** used primarily for diagnosing bacterial infections, particularly sepsis, and is not involved in amyloidogenesis. - It does not relate to amyloidosis and is not a component of amyloid fibrils. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 266-267.

Q: According to WHO/ISN classification, which class of lupus nephritis shows a membranous pattern in SLE?

Membranous pattern. ***Membranous pattern*** - This corresponds to **Class V lupus nephritis** in the WHO/ISN classification, characterized by widespread immune complex deposition along the **glomerular basement membrane (GBM)**. - The subepithelial immune deposits lead to GBM thickening, creating the characteristic membranous pattern on light microscopy. - This pattern resembles idiopathic membranous nephropathy but occurs in the context of SLE. *Mesangial pattern involvement* - This refers to **Class I (minimal mesangial LN)** or **Class II (mesangial proliferative LN)**, where immune deposits are primarily confined to the mesangium. - There is minimal or no involvement of the glomerular capillary walls, distinguishing it from the membranous pattern. *Diffuse proliferative pattern* - This is **Class IV lupus nephritis**, the most severe form characterized by widespread **endocapillary and/or extracapillary proliferation** involving ≥50% of glomeruli. - The primary feature is cellular proliferation (mesangial, endocapillary, epithelial crescents), not the subepithelial immune deposits typical of membranous pattern. *Focal proliferative pattern* - This corresponds to **Class III lupus nephritis**, involving **endocapillary or extracapillary proliferation** in <50% of glomeruli. - Distinguished by focal (not diffuse) involvement and active proliferation rather than the membranous pattern seen in Class V.

Q: The MOST COMMON cause of concentric hypertrophy of left ventricle is?

Hypertension. ***Hypertension*** - Chronic **hypertension** is the most common cause of **pressure overload** on the left ventricle, leading to concentric hypertrophy [1]. - In response to the increased afterload, the ventricular wall thickens uniformly inward, reducing the chamber size while maintaining normal wall stress [2]. - Due to its high prevalence (30-40% of adults), hypertension is epidemiologically the most frequent cause of concentric LVH [1]. *Aortic stenosis* - While **aortic stenosis** is the classic pathological cause of **pressure overload** and concentric hypertrophy [2], **hypertension** is more prevalent in the population. - Aortic stenosis causes fixed outflow obstruction, leading to significant pressure work for the left ventricle. - This is the second most common cause but occurs in only 2-5% of elderly patients. *Mitral stenosis* - **Mitral stenosis** primarily causes pressure overload on the **left atrium** and **pulmonary circulation**, not the left ventricle. - It doesn't typically lead to **left ventricular hypertrophy** directly; instead, it causes left atrial enlargement and right ventricular hypertrophy. *Aortic regurgitation* - **Aortic regurgitation** results in **volume overload** of the left ventricle due to blood flowing back into the chamber during diastole. - This typically leads to **eccentric hypertrophy**, where the chamber dilates and the wall thickens proportionally, rather than concentric hypertrophy [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, pp. 560-562. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, p. 536.

Q: Most common cause of death in SLE in children

Anemia and infections. ***Anemia and infections*** - **Infections** are a leading cause of death in pediatric SLE patients, often due to immunosuppression from the disease itself or its treatment. Although pediatric Systemic Lupus Erythematosus (SLE) is not a primary immune deficiency, the susceptibility to encapsulated bacteria and recurrent infections seen in primary B- and T-lymphocyte deficiencies mirrors the infection risks managed in these patients [1]. - **Anemia** can contribute to overall morbidity and mortality, although it is less directly a cause of death than severe infections or organ failure. *Lupus nephritis* - While **lupus nephritis** is a common and severe manifestation of SLE in children and a major cause of morbidity, particularly long-term kidney failure, it is not the most frequent immediate cause of death. - Advancements in treatment for nephritis have improved prognosis, shifting the leading cause of mortality to other factors. *Lupus cerebritis* - **Lupus cerebritis** (neuropsychiatric SLE) can be life-threatening, causing seizures, stroke, or psychosis, but it is less common as the primary cause of death compared to infections. - Its presence usually indicates severe disease requiring intensive treatment, but not the most common direct cause of death. *Libman sacks endocarditis* - **Libman-Sacks endocarditis** involves sterile vegetations on heart valves and is a known complication of SLE, but it rarely causes acute mortality in children. - It is more often associated with chronic complications like valvular dysfunction or a source of emboli rather than being the most common cause of death.

Q: What is the primary action observed in the withdrawal reflex?

Flexion. ***Flexion*** - The **withdrawal reflex** is a protective reflex that causes the affected limb to **flex** and withdraw from a painful stimulus. - This **flexion** is mediated by the contraction of flexor muscles and relaxation of extensor muscles, moving the limb away from danger. *Extension* - **Extension** is the opposite of flexion and would move the limb closer to or maintain its position relative to the painful stimulus. - This action is typically observed in the **crossed extensor reflex**, where the contralateral limb extends to support the body, not in the direct withdrawal of the stimulated limb. *Flexion followed by extension* - While **flexion** is the primary action, it is not typically followed immediately by extension within the same limb in a simple withdrawal reflex. - If a coordinated movement were to occur, such as shifting weight, the **crossed extensor reflex** would involve extension in the opposite limb. *Not applicable* - The withdrawal reflex involves a clear and defined muscle action which is **flexion**, making "not applicable" incorrect. - This reflex is a fundamental component of the nervous system's response to noxious stimuli.

Q: Which type of muscle fibers has fewer mitochondria?

Type IIb fibers (Fast-twitch fibers). ***Type IIb fibers (Fast-twitch fibers)*** - These fibers rely primarily on **anaerobic glycolysis** for ATP production, which is a less efficient process than aerobic respiration and therefore requires fewer mitochondria. - Their primary function is rapid, powerful contractions over short durations, leading to quick fatigue. *Type IIa fibers* - These fibers are **fast-twitch oxidative-glycolytic** fibers, meaning they have a moderate number of mitochondria to support both aerobic and anaerobic metabolism. - They are capable of generating strong contractions and are more fatigue-resistant than Type IIb fibers but less so than Type I fibers. *Type I fibers (Red fibers)* - Known as **slow-twitch oxidative fibers**, they have a high density of mitochondria to support continuous **aerobic respiration** for sustained, low-intensity contractions. - Their rich blood supply and high myoglobin content give them their characteristic red color and make them highly fatigue-resistant. *Type IIx fibers (Intermediate fibers)* - These fibers are very similar to Type IIb fibers in their metabolic profile, often considered an intermediate or even functionally equivalent type depending on the species. - They also primarily utilize **anaerobic glycolysis** and have a relatively low mitochondrial content, making them prone to fatigue.

Question 1

Which of the following enzymes is classified as a serine protease?

Accepted Answer

Trypsin

Answer

Pepsin

Answer

Carboxypeptidase

Answer

None of the options

Question 2

Rate limiting step in pyrimidine synthesis?

Accepted Answer

Carbamoyl phosphate synthase-II

Answer

Aspartate transcarbamoylase (ATCase)

Answer

Dihydroorotate dehydrogenase

Answer

Dihydro-orotase

Question 3

Glucagon stimulates

Accepted Answer

Gluconeogenesis

Answer

Glycogenesis

Answer

Fatty acid synthesis

Answer

Glycolysis

Question 4

Which of the following statements regarding collagen synthesis is incorrect?

Accepted Answer

Hydroxylation of proline occurs in Golgi apparatus

Answer

Hydroxylation of lysine occurs in ER

Answer

Synthesized in ribosomes as preprocollagen

Answer

Triple helix assembly occurs in ER

Question 5

What is the major fibril protein associated with Primary Amyloidosis?

Accepted Answer

Immunoglobulin Light Chain

Answer

Amyloid Associated protein (AA)

Answer

Procalcitonin (PCT)

Answer

Transthyretin (TTR)

Question 6

According to WHO/ISN classification, which class of lupus nephritis shows a membranous pattern in SLE?

Accepted Answer

Membranous pattern

Answer

Diffuse proliferative pattern

Answer

Mesangial pattern involvement

Answer

Focal proliferative pattern

Question 7

The MOST COMMON cause of concentric hypertrophy of left ventricle is?

Accepted Answer

Hypertension

Answer

Aortic stenosis

Answer

Mitral stenosis

Answer

Aortic regurgitation

Question 8

Most common cause of death in SLE in children

Accepted Answer

Anemia and infections

Answer

Libman sacks endocarditis

Answer

Lupus cerebritis

Answer

Lupus nephritis

Question 9

What is the primary action observed in the withdrawal reflex?

Accepted Answer

Flexion

Answer

Extension

Answer

Flexion followed by extension

Answer

Not applicable

Question 10

Which type of muscle fibers has fewer mitochondria?

Accepted Answer

Type IIb fibers (Fast-twitch fibers)

Answer

Type I fibers (Red fibers)

Answer

Type IIa fibers

Answer

Type IIx fibers (Intermediate fibers)

NEET-PG 2013

Biochemistry

Pathology

Pediatrics

Physiology