FMGE 2025 - Biochemistry Practice Questions

Q: Which of the following amino acids primarily acts as a buffer in blood due to its ability to accept and donate protons at physiological pH?

Histidine. ***Histidine***- The side chain of **histidine**, the **imidazole group**, has a pKa of approximately 6.0, which is close to the physiological pH of **7.4**, making it an effective buffer.- This property is especially vital for the **buffering capacity of hemoglobin** in red blood cells, contributing significantly to pH homeostasis (Bohr effect).*Arginine*- Arginine possesses a **guanidinium group** in its side chain with a very high pKa (~12.5).- This high pKa means its side chain is almost always positively charged and protonated at physiological pH, rendering it ineffective as a physiological **acid-base buffer**.*Tryptophan*- Tryptophan has a large, non-polar **indole ring** side chain, which is chemically inert and lacks an ionizable group within the physiological pH range.- Since it cannot accept or donate protons near pH 7.4, it does not contribute to the **buffering system** of the blood.*Tyrosine*- Tyrosine contains a **phenolic hydroxyl group** with a pKa of approximately 10.5.- Because its pKa is significantly higher than physiological pH, it is largely neutral and incapable of mediating proton exchange effectively in the **blood plasma**.

Q: During starvation, which of the following hormones is primarily produced to maintain blood glucose levels?

Glucagon. ***Glucagon*** - Glucagon is the **primary counter-regulatory hormone** secreted by the pancreatic **alpha cells** in response to hypoglycemia during starvation. - It acts mainly on the liver to stimulate rapid glucose release through **glycogenolysis** and sustain long-term glucose production via **gluconeogenesis**. - Glucagon levels rise significantly within hours of fasting and remain elevated throughout prolonged starvation. *Insulin* - Insulin is an **anabolic hormone** secreted in response to high blood glucose (hyperglycemia) to promote storage and glucose uptake, thus lowering blood glucose levels. - During starvation, insulin secretion is characteristically **suppressed** to minimize glucose uptake by peripheral tissues and conserve it for the brain. *Cortisol* - Cortisol is a **glucocorticoid stress hormone** that does increase during prolonged starvation and contributes to gluconeogenesis and protein catabolism. - However, **glucagon is the primary and most rapid responder** to falling blood glucose levels, making it the correct answer to this question. *Somatostatin* - Somatostatin is a **paracrine inhibitor** secreted by pancreatic delta cells that locally suppresses the release of both insulin and glucagon. - While it modulates islet function, it is not the primary hormone responsible for mobilizing stored fuels and **raising blood glucose** during periods of fasting.

Q: The development of cataracts in patients with Diabetes Mellitus is primarily due to the accumulation of which of the following substances in the lens?

Sorbitol. ***Correct Option: Sorbitol*** - In the setting of chronic **hyperglycemia**, excess glucose is converted into **sorbitol** by the enzyme **aldose reductase** via the polyol pathway. - Sorbitol is poorly transported out of the lens cells and its accumulation creates an internal **osmotic gradient**, drawing water into the lens, which leads to cell swelling, lens fiber disruption, and eventual **cataract** formation. - This is the **primary mechanism** of diabetic cataract development. *Incorrect Option: Galactitol* - **Galactitol** (also known as dulcitol) is the specific sugar alcohol that accumulates when there is deficiency of **galactokinase** or **galactose-1-phosphate uridyltransferase**. - Its accumulation is characteristic of **galactosemia**, where high galactose levels lead to its formation via aldose reductase, causing congenital cataracts in that condition, not diabetic cataracts. *Incorrect Option: Fructose* - While **fructose** is produced from sorbitol by sorbitol dehydrogenase in the polyol pathway, it is readily metabolized and does not accumulate in lens tissue. - Fructose itself does not cause osmotic damage or cataract formation in diabetes. *Incorrect Option: Mannitol* - **Mannitol** is a hexahydric sugar alcohol primarily used pharmacologically as an osmotic diuretic for conditions like **cerebral edema**. - It is not an endogenous product of glucose metabolism in the lens and is not associated with diabetic cataract formation.

Q: A patient has a long arm span, hypermobile joints and ectopia lentis. What is the defective protein?

Fibrillin-1. ***Fibrillin-1***- **Marfan syndrome**, characterized by a **long arm span (arachnodactyly)**, **joint hypermobility**, and superiorly displaced **ectopia lentis (hypermobile lens)**, results from mutations in the *FBN1* gene, which codes for **Fibrillin-1**.- **Fibrillin-1** is a critical component of connective tissue **microfibrils** found in the suspensory ligaments of the eye, periosteum, and aortic media, explaining the systemic clinical manifestations.*Collagen*- Defects in **Collagen** (e.g., Type I, III, or V) are associated with **Osteogenesis Imperfecta** (brittle bones) or **Ehlers-Danlos syndrome** (severe skin and joint hyperlaxity) [1].- While Ehlers-Danlos syndrome can cause joint hypermobility, the classic combination with **ectopia lentis** strongly differentiates Marfan syndrome from primary collagenopathies [1].*Elastin*- Mutations in the **Elastin** gene (ELN) are primarily linked to conditions like **Williams syndrome**, which typically presents with **supravalvular aortic stenosis** and characteristic facial features [2].- **Elastin** provides rubber-like properties to tissues, but its primary defect does not explain the specific loss of structural integrity leading to superiorly displaced **ectopia lentis** seen in Marfan syndrome [2].*Keratin*- **Keratin** proteins are intermediate filaments crucial for mechanical stability in **epithelial cells** (skin, hair, nails).- Defects in keratin primarily cause **epidermolysis bullosa** or various forms of **ichthyosis** (skin blistering/scaling), and do not lead to the systemic skeletal or ocular connective tissue anomalies observed here.

Q: A patient presents with multiple colonic polyps and has been diagnosed with colorectal carcinoma. There is a strong family history of Hereditary Non-Polyposis Colorectal Cancer (HNPCC). Which DNA repair mechanism is most likely defective in this condition?

Mismatch repair. ***Mismatch repair***- **HNPCC (Lynch syndrome)** is caused by inherited germline mutations in genes (e.g., *MLH1*, *MSH2*) that are responsible for the **mismatch repair (MMR)** pathway. - The failure of MMR leads to the accumulation of errors, specifically in repetitive DNA sequences, resulting in **microsatellite instability** which drives carcinogenesis.*Nucleotide excision repair*- This mechanism repairs bulky helix-distorting lesions in DNA, most commonly **pyrimidine dimers** caused by UV radiation. - A classic disease associated with defective **NER** is **Xeroderma Pigmentosum**, which presents with extreme sun sensitivity and a high risk of skin cancers.*Base excision repair*- This pathway primarily corrects small, non-helix distorting damage, such as oxidized or alkylated bases, utilizing enzymes like **DNA glycosylases**.- While fundamental for DNA maintenance, primary defects in BER are not the underlying cause of pathogenesis in **Lynch syndrome**.*Double-strand break repair*- This mechanism repairs severe damage where the entire DNA helix is broken, typically via **Homologous Recombination (HR)** or **Non-Homologous End Joining (NHEJ)**. - Defects in HR are often linked to hereditary breast and ovarian cancers (e.g., *BRCA1*/ *BRCA2* mutations) and are distinct from the pathogenesis of **HNPCC**.

Q: A man presents with high VLDL and chylomicrons, with eruptive xanthomas. What is the most likely diagnosis?

Familial hypertriglyceridemia. ***Familial hypertriglyceridemia (Type IV or V hyperlipidemia)*** - This condition presents with **elevated VLDL and chylomicrons**, causing severe hypertriglyceridemia [1] - **Eruptive xanthomas** are pathognomonic for triglyceride levels >1000 mg/dL, appearing as small yellow papules on extensor surfaces and buttocks - Patients are at risk for **acute pancreatitis** due to extreme triglyceride elevation - Treatment includes **fibrates, omega-3 fatty acids, and strict dietary fat restriction** [2] *Familial hyperlipidemia* - This is a non-specific term that could refer to any inherited lipid disorder - Not a precise diagnostic entity used in clinical practice *Familial dysbetalipoproteinemia (Type III)* - Characterized by elevated **IDL (intermediate-density lipoproteins)**, not primarily VLDL + chylomicrons [1] - Presents with **palmar xanthomas (yellowish discoloration of palmar creases)** and tuberoeruptive xanthomas [1] - Different lipid pattern and xanthoma distribution than described *Familial hypercholesterolemia* - Characterized by elevated **LDL cholesterol**, not triglycerides [1] - Presents with **tendinous xanthomas, xanthelasma, and corneal arcus** [1] - Does not cause eruptive xanthomas or chylomicronemia

Q: What is the primary protein that binds to thyroxine?

Thyroxine-binding globulin. ***Thyroxine-binding globulin*** - **TBG** is a single chain glycoprotein that is the primary transporter, binding approximately 70-80% of circulating **T4** (thyroxine) and a smaller proportion of T3. - It serves as a high-affinity reservoir, maintaining hormonal homeostasis and contributing to the long **half-life** of T4. *Follistatin* - **Follistatin** binds and inhibits the activity of **activin** and is primarily involved in regulating the release of **FSH** (follicle-stimulating hormone) from the pituitary. - It has no functional role in the systemic transport or binding of thyroid hormones. *Transthyretin* - **Transthyretin** (or prealbumin) is the second most abundant carrier, binding about 10-15% of circulating T4, and is the principal carrier of **retinol** (Vitamin A). - Although it transports T4, its binding affinity and capacity are significantly lower than those of **TBG**, thus it is not the primary binder. *Transferrin* - **Transferrin** is the main plasma protein responsible for transporting **ferric iron (Fe3+)** throughout the body. - Its binding specificity is strictly for iron and does not involve the transport of **thyroxine**.

Q: Which vitamin deficiency can lead to lactic acidosis?

Thiamine. ***Thiamine*** - **Thiamine (Vitamin B1)** is a crucial cofactor for the enzyme **pyruvate dehydrogenase (PDH)**, which converts **pyruvate** to **acetyl-CoA** to enter the Krebs cycle. - When thiamine is deficient, pyruvate cannot be processed efficiently, leading to its accumulation and subsequent shunting into **lactate** via **lactate dehydrogenase**, thus causing **lactic acidosis**, characteristic of **beriberi**. *Riboflavin* - Riboflavin (Vitamin B2) is a precursor for the coenzymes **FAD** and **FMN**, essential for redox reactions, but its deficiency symptoms mainly involve the oral cavity and skin. - Deficiency leads to **angular cheilitis**, **glossitis**, and **corneal vascularization**, and is not the direct cause of severe lactic acidosis. *Niacin* - Niacin (Vitamin B3) is a precursor for **NAD+** and **NADP+**, vital for many metabolic reactions. - Deficiency causes **pellagra**, characterized by the 3 D's: **Dermatitis**, **Diarrhea**, and **Dementia**, not primarily lactic acidosis. *Biotin* - Biotin (Vitamin B7) acts as a coenzyme for **carboxylase enzymes** (e.g., pyruvate carboxylase, acetyl-CoA carboxylase), which are necessary for gluconeogenesis and fatty acid synthesis. - While **pyruvate carboxylase** uses biotin, thiamine deficiency specifically impairs the key step of converting pyruvate to acetyl-CoA (via PDH complex), making it the primary cause of vitamin-deficiency-related lactic acidosis.

Q: Which of the following enzymes is responsible for the immortality of cancer cells?

Telomerase. ***Telomerase*** **Telomerase** is a **ribonucleoprotein enzyme** that adds repetitive DNA sequences (TTAGGG) to the ends of chromosomes (**telomeres**), counteracting the natural shortening that occurs during cell division. The activation of **telomerase** is characteristic of most cancer cells, granting them the ability to divide indefinitely, fulfilling the hallmark of **unlimited replicative potential** (immortality). *Topoisomerase* This enzyme is crucial for relieving the **torsional strain** (supercoiling) in the DNA helix that arises ahead of the replication fork or during transcription. Its primary role is managing DNA structure and integrity, not determining cellular lifespan or **immortality** through **telomere** maintenance. *Helicase* **Helicases** are motor proteins that use energy from ATP hydrolysis to **unwind nucleic acid duplexes** (like the DNA double helix) in fundamental processes such as DNA replication, repair, and transcription. While essential for replication, it does not prevent the gradual loss of terminal DNA sequences (**telomeres**) required for cancer cell immortality. *RNA polymerase* This enzyme is responsible for **transcription**, the process of synthesizing an **RNA molecule** from a DNA template. Its function is focused on gene expression (protein synthesis) and is not directly involved in maintaining the length of **telomeres** or conferring replicative immortality upon cells.

Question 1

Which of the following amino acids primarily acts as a buffer in blood due to its ability to accept and donate protons at physiological pH?

Accepted Answer

Histidine

Answer

Arginine

Answer

Tryptophan

Answer

Tyrosine

Question 2

During starvation, which of the following hormones is primarily produced to maintain blood glucose levels?

Accepted Answer

Glucagon

Answer

Insulin

Answer

Cortisol

Answer

Somatostatin

Question 3

The development of cataracts in patients with Diabetes Mellitus is primarily due to the accumulation of which of the following substances in the lens?

Accepted Answer

Sorbitol

Answer

Galactitol

Answer

Fructose

Answer

Mannitol

Question 4

A patient has a long arm span, hypermobile joints and ectopia lentis. What is the defective protein?

Accepted Answer

Fibrillin-1

Answer

Keratin

Answer

Elastin

Answer

Collagen

Question 5

A patient presents with multiple colonic polyps and has been diagnosed with colorectal carcinoma. There is a strong family history of Hereditary Non-Polyposis Colorectal Cancer (HNPCC). Which DNA repair mechanism is most likely defective in this condition?

Accepted Answer

Mismatch repair

Answer

Double-strand break repair

Answer

Nucleotide excision repair

Answer

Base excision repair

Question 6

A man presents with high VLDL and chylomicrons, with eruptive xanthomas. What is the most likely diagnosis?

Accepted Answer

Familial hypertriglyceridemia

Answer

Familial hyperlipidemia

Answer

Familial dysbetalipoproteinemia

Answer

Familial hypercholesterolemia

Question 7

What is the primary protein that binds to thyroxine?

Accepted Answer

Thyroxine-binding globulin

Answer

Follistatin

Answer

Transthyretin

Answer

Transferrin

Question 8

A breastfed infant presents with lethargy, hepatomegaly, and cataracts. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Accepted Answer

Galactose-1-phosphate uridyltransferase (GALPUT)

Answer

Galactokinase

Answer

Aldolase B

Answer

Fructokinase

Question 9

Which vitamin deficiency can lead to lactic acidosis?

Accepted Answer

Thiamine

Answer

Riboflavin

Answer

Niacin

Answer

Biotin

Question 10

Which of the following enzymes is responsible for the immortality of cancer cells?

Accepted Answer

Telomerase

Answer

RNA polymerase

Answer

Helicase

Answer

Topoisomerase

FMGE 2025 — Biochemistry