FMGE 2025 - Biochemistry PYQs Practice Questions

Q: Which of the following is excreted in the urine of patients with pentosuria?

L-Xylulose. ***L-Xylulose***- Pentosuria is a benign, autosomal recessive disorder caused by a deficiency in the enzyme **L-xylulose reductase** (or xylitol dehydrogenase), which is part of the **glucuronic acid pathway (GAA)**.- This deficiency prevents the conversion of **L-xylulose** (a pentose intermediate) to xylitol, leading to the accumulation and subsequent excretion of L-xylulose in the urine.*D-Ribose*- D-Ribose is an **aldopentose** sugar crucial for forming the backbone of **RNA**, **ATP**, and various coenzymes (e.g., NAD, FAD).- Its metabolism is primarily handled by the **pentose phosphate pathway**; its excretion is not the hallmark of the enzyme deficiency in pentosuria.*D-Ribulose*- D-Ribulose is a **ketopentose** that plays a key role as an intermediate in the **pentose phosphate pathway** (PPP), specifically forming xylulose 5-phosphate.- Although it is a pentose, its accumulation and excretion are not characteristic findings of the defective **L-xylulose reductase** enzyme in pentosuria.*D-Lactose*- D-Lactose is a **disaccharide** composed of glucose and galactose, derived from milk, and is not a pentose sugar.- Lactosuria (lactose excretion) is typically related to high dietary intake or severe disorders affecting **lactose metabolism** or hepatic function, which is unrelated to the biochemical defect causing pentosuria. [Practice more Biochemistry PYQs on OnCourse]

Q: A patient with a long-standing history of diabetes presents with cataracts. Which of the following metabolic products is primarily responsible for cataract formation in this condition?

Sorbitol. ***Sorbitol*** - When blood glucose is elevated in diabetes, the enzyme **aldose reductase** converts excess glucose into **sorbitol** (a polyol) in tissues like the lens, which do not require insulin for glucose uptake. - Sorbitol is poorly permeable across cell membranes. Its accumulation creates a significant **osmotic gradient** within the lens fibers, causing water influx and subsequent lens swelling and opacification (cataract formation). *Mannitol* - **Mannitol** is a polyol often used as an osmotic diuretic in clinical settings to reduce cerebral edema or intraocular pressure. - Although chemically similar to sorbitol, mannitol accumulation is not the primary mechanism of **cataract formation** specifically linked to chronic hyperglycemia. *Galactitol* - **Galactitol** is NOT responsible for cataract formation in diabetes, but is specifically responsible for cataracts in individuals with **galactosemia** (inability to metabolize galactose). - It is formed from **galactose** via aldose reductase, making it the incorrect metabolite in a patient with diabetes-related cataracts. *Fructose* - **Fructose** is formed when sorbitol is further metabolized by the enzyme **sorbitol dehydrogenase**. - While present in the polyol pathway, **sorbitol** itself is the product whose high intracellular concentration and osmotic activity is the direct cause of the diabetic cataract. [Practice more Biochemistry PYQs on OnCourse]

Q: A patient presents with gum bleeding, petechiae, poor wound healing, and other related symptoms. Which of the following vitamin deficiencies is most likely responsible for this presentation?

Vitamin C. ***Vitamin C*** - Vitamin C (ascorbic acid) is essential for the hydroxylation of **proline** and **lysine** residues during collagen synthesis, crucial for maintaining the integrity of blood vessels and skin. - Deficiency results in **scurvy**, characterized by defective collagen, leading to fragile capillaries (manifesting as **gum bleeding** and **petechiae**) and impaired **wound healing**. *Vitamin A* - Deficiency is primarily associated with visual disturbances, most notably **night blindness** (**nyctalopia**), and **xerophthalmia** (dry eyes). - Its main roles involve vision, epithelial differentiation, and immune function, not the stability of subendothelial collagen required to prevent bleeding tendencies described here. *Vitamin B6* - Pyridoxine deficiency symptoms include **seborrheic dermatitis**, **stomatitis**, **cheilosis**, and neurological issues like **peripheral neuropathy** and seizures in infants. - While involved in numerous metabolic pathways, its deficiency is not responsible for the characteristic symptoms of capillary fragility (petechiae and bleeding). *Vitamin B3* - Niacin deficiency causes **pellagra**, classically identified by the "3 D's": **Dermatitis** (photosensitive rash), **Diarrhea**, and **Dementia**. - Although pellagra involves epithelial changes (dermatitis), it does not typically present with the severe vascular fragility, gum manifestations, and poor generalized wound healing seen in scurvy. [Practice more Biochemistry PYQs on OnCourse]

Q: A patient presents with arthritis, and arthroscopy reveals blackening of the joints. Which enzyme deficiency is most likely responsible for this condition?

Homogentisate oxidase. ***Homogentisate oxidase***- Deficiency of this enzyme causes **Alkaptonuria**, resulting in the accumulation of **homogentisic acid**, which oxidizes into dark polymeric pigment called **ochronotic pigment**.- This pigment deposits in cartilage and connective tissues (**Ochronosis**), leading to severe destructive arthritis, especially in large joints, and causing the characteristic **blackening of joint tissues** observed on arthroscopy.*Branched-chain alpha-ketoacid dehydrogenase*- Deficiency of this enzyme causes **Maple Syrup Urine Disease (MSUD)**, leading to the accumulation of **branched-chain amino acids** (leucine, isoleucine, valine).- MSUD is characterized by severe neurologic dysfunction and a distinctive maple syrup odor in the urine, not chronic arthritis or joint pigmentation.*Tyrosinase*- This enzyme is crucial for the synthesis of **melanin** from tyrosine.- Deficiency results in conditions like **Albinism**, characterized by hypopigmentation of the skin, hair, and eyes, which is distinct from ochronotic arthritis.*Fumarase*- Deficiency of this enzyme (Fumarate hydratase) impairs the **Krebs cycle** (TCA cycle).- Clinical features include severe intellectual disability, microcephaly, and metabolic acidosis, without the development of ochronosis or black joints. [Practice more Biochemistry PYQs on OnCourse]

Q: A patient with a history of strenuous exercise skipped a meal and later consumed alcohol at a party. Alcohol inhibits which of the following biochemical processes, potentially leading to hypoglycemia?

Gluconeogenesis. ***Gluconeogenesis*** - Alcohol metabolism by **alcohol dehydrogenase** and **aldehyde dehydrogenase** generates a massive influx of **NADH**, drastically increasing the hepatic NADH/NAD+ ratio. - This high NADH/NAD+ ratio shifts metabolic intermediates, inhibiting key enzymes (e.g., **lactate dehydrogenase** and **malate dehydrogenase**) required to convert precursors like lactate and amino acids into glucose, leading to failure of new glucose synthesis and subsequent hypoglycemia. *Glycogenolysis* - This process, the breakdown of stored **glycogen** into glucose, is not directly inhibited by the increased NADH ratio produced during alcohol metabolism. - In fact, the initial phase of alcohol-induced hypoglycemia relies on the depletion of pre-existing glycogen stores, which is accelerated by factors like strenuous exercise or fasting. *Hexose Monophosphate (HMP) Shunt* - The primary function of the HMP shunt is to produce **NADPH** (for reductive biosynthesis and antioxidant defense) and five-carbon sugars (**ribose-5-phosphate**) for nucleotide synthesis. - Inhibition of the HMP shunt alters cell redox status and nucleotide production but does not directly impair blood glucose maintenance or cause acute hypoglycemia. *Ketone Body Formation* - Alcohol metabolism actually tends to **inhibit** ketone body formation because the high NADH ratio inhibits **beta-oxidation** of fatty acids in the liver, which is required to produce the Acetyl-CoA necessary for ketogenesis. - While ketone formation is inhibited, this is a consequence of high NADH, and the hypoglycemia itself results from the inability to synthesize glucose via gluconeogenesis, not the absence of ketones. [Practice more Biochemistry PYQs on OnCourse]

Question 1

Which of the following is excreted in the urine of patients with pentosuria?

Accepted Answer

L-Xylulose

Answer

D-Lactose

Answer

D-Ribulose

Answer

D-Ribose

Question 2

A patient with a long-standing history of diabetes presents with cataracts. Which of the following metabolic products is primarily responsible for cataract formation in this condition?

Accepted Answer

Sorbitol

Answer

Galactitol

Answer

Mannitol

Answer

Fructose

Question 3

A patient presents with gum bleeding, petechiae, poor wound healing, and other related symptoms. Which of the following vitamin deficiencies is most likely responsible for this presentation?

Accepted Answer

Vitamin C

Answer

Vitamin B3

Answer

Vitamin B6

Answer

Vitamin A

Question 4

A patient presents with arthritis, and arthroscopy reveals blackening of the joints. Which enzyme deficiency is most likely responsible for this condition?

Accepted Answer

Homogentisate oxidase

Answer

Fumarase

Answer

Tyrosinase

Answer

Branched-chain alpha-ketoacid dehydrogenase

Question 5

A patient with a history of strenuous exercise skipped a meal and later consumed alcohol at a party. Alcohol inhibits which of the following biochemical processes, potentially leading to hypoglycemia?

Accepted Answer

Gluconeogenesis

Answer

Glycogenolysis

Answer

Ketone Body Formation

Answer

Hexose Monophosphate (HMP) Shunt

Question 6

Which of the following accumulates in the mucopolysaccharidosis associated with Natowicz disease?

Accepted Answer

Hyaluronic acid

Answer

Dermatan sulfate

Answer

Heparan sulfate

Answer

Keratan sulfate

Question 7

Which of the following enzymes is dependent on Vitamin C for its activity?

Accepted Answer

Prolyl hydroxylase

Answer

Lysyl oxidase

Answer

Lysyl dehydrogenase

Answer

Hydroxyprolyl kinase

Question 8

Which of the following enzymes is typically elevated in prostate cancer?

Accepted Answer

Acid phosphatase

Answer

Alpha-fetoprotein

Answer

Lactate dehydrogenase

Answer

Alkaline phosphatase

Question 9

Which deficiency is most likely in a patient with delayed wound healing and alopecia?

Accepted Answer

Zinc

Answer

Calcium

Answer

Copper

Answer

Vitamin D

Question 10

A disease with mitochondrial inheritance is inherited from which family member?

Accepted Answer

Mother

Answer

Father

Answer

Both mother and father

Answer

Grandmother (paternal)

FMGE 2025 — Biochemistry