Genetic Disorders and Biochemical Pathology — MCQs

A 6-month-old boy presented with failure to thrive. Laboratory investigations revealed hypoglycemia and elevated urinary glutamine and uracil levels. Gastric tube feeding was poorly tolerated. The child became comatose, and after parenteral dextrose administration, recovered within 24 hours. Which of the following enzymes is defective?

Q320Medium

What is the most common enzyme deficiency leading to porphyria in humans?

Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs

Question 311: Which statement is false regarding Gilbert syndrome?

A. Male predominant
B. UGT activity is 10-30% of normal
C. Histology normal
D. Mild conjugated hyperbilirubinemia (Correct Answer)

Explanation: **Explanation:** Gilbert Syndrome is a common, benign autosomal recessive condition characterized by reduced activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. **Why Option D is the Correct (False) Statement:** Gilbert syndrome is characterized by **unconjugated hyperbilirubinemia**, not conjugated. The defect lies in the conjugation process itself; because the UGT1A1 enzyme is deficient, free (unconjugated) bilirubin cannot be converted into water-soluble conjugated bilirubin. Therefore, levels of indirect bilirubin rise, typically remaining <3 mg/dL. **Analysis of Other Options:** * **Option A (Male predominant):** This is true. The condition is more frequently diagnosed in males, likely due to a higher daily production of bilirubin in men compared to women. * **Option B (UGT activity is 10-30%):** This is true. In Gilbert syndrome, there is a molecular defect (usually a TATAA box mutation) that reduces enzyme expression to roughly 10–30% of normal levels. (Note: In Crigler-Najjar Type II, activity is <10%, and in Type I, it is 0%). * **Option C (Histology normal):** This is true. Liver biopsy is not indicated, but if performed, the liver architecture appears completely normal, distinguishing it from other causes of jaundice like hepatitis or cirrhosis. **High-Yield Clinical Pearls for NEET-PG:** * **Triggers:** Jaundice is typically intermittent and provoked by **stress, fasting (starvation), infection, or strenuous exercise.** * **Diagnosis:** Suspect Gilbert in an asymptomatic patient with isolated unconjugated hyperbilirubinemia and normal LFTs (ALT, AST, ALP). * **Rifampicin Test:** Administration of Rifampicin can increase serum bilirubin levels in these patients, serving as a provocative test. * **Management:** No treatment is required; it is a "state" rather than a disease.

Question 312: Genomic imprinting is observed in which of the following conditions?

A. Klinefelter's syndrome
B. Down's syndrome
C. Angelman syndrome (Correct Answer)
D. Hydatidiform mole

Explanation: **Explanation:** **Genomic Imprinting** is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. This means one allele is "silenced" (via DNA methylation) depending on whether it was inherited from the mother or the father. **Why Angelman Syndrome is Correct:** Angelman syndrome and Prader-Willi syndrome are the classic examples of imprinting defects involving **Chromosome 15 (q11-q13)**. * **Angelman Syndrome ("Happy Puppet"):** Occurs when the **maternal** allele (UBE3A gene) is deleted or mutated, while the paternal allele is normally imprinted (silenced). * **Prader-Willi Syndrome:** Occurs when the **paternal** allele is deleted, while the maternal allele is silenced. **Analysis of Incorrect Options:** * **A & B (Klinefelter’s and Down’s Syndrome):** These are **Aneuploidies** (numerical chromosomal aberrations) caused by nondisjunction during meiosis. They do not involve gene silencing via imprinting. * **D (Hydatidiform Mole):** While a complete mole involves only paternal chromosomes (and is an example of genomic imbalance), it is a gestational trophoblastic disease rather than a classic "imprinting disorder" syndrome like Angelman. **High-Yield Clinical Pearls for NEET-PG:** * **Uniparental Disomy (UPD):** A common cause of imprinting disorders where an individual inherits two copies of a chromosome from one parent and none from the other. * **Angelman Presentation:** Inappropriate laughter, seizures, ataxia, and severe intellectual disability. * **Prader-Willi Presentation:** Hyperphagia (obesity), hypogonadism, and hypotonia. * **Other Imprinting Disorders:** Beckwith-Wiedemann Syndrome (Chromosome 11p15).

Question 313: Age-related dementia has been associated with increased levels of which substance?

A. Homocysteine (Correct Answer)
B. Cysteine
C. Taurine
D. Methionine

Explanation: ### Explanation **Correct Answer: A. Homocysteine** **Mechanism and Clinical Significance:** Elevated levels of **Homocysteine (Hyperhomocysteinemia)** are strongly associated with neurodegenerative conditions, including age-related dementia and Alzheimer’s disease. Homocysteine acts as a potent neurotoxin through several mechanisms: 1. **Vascular Damage:** It promotes endothelial dysfunction and atherosclerosis of cerebral blood vessels, leading to chronic hypoperfusion. 2. **Excitotoxicity:** It acts as an agonist at NMDA receptors, leading to calcium influx and neuronal apoptosis. 3. **Oxidative Stress:** It increases the production of reactive oxygen species (ROS), damaging neuronal DNA. 4. **Amyloid Accumulation:** High levels are linked to increased deposition of beta-amyloid plaques and tau hyperphosphorylation. **Analysis of Incorrect Options:** * **B. Cysteine:** This is a non-essential amino acid derived from homocysteine via the cystathionine pathway. It is a precursor for glutathione (an antioxidant) and is generally considered neuroprotective rather than a risk factor for dementia. * **C. Taurine:** An amino acid derivative that functions as an inhibitory neurotransmitter and osmolyte. It has cytoprotective properties and is often studied for its potential role in *preventing* neurodegeneration. * **D. Methionine:** An essential amino acid that serves as the precursor to S-adenosylmethionine (SAMe). While homocysteine is derived from methionine metabolism, methionine itself is not the primary biomarker for dementia risk. **NEET-PG High-Yield Pearls:** * **Vitamin Deficiencies:** Hyperhomocysteinemia is often caused by deficiencies in **Vitamin B12 (Cobalamin), B9 (Folate), and B6 (Pyridoxine)**, as these are essential cofactors for its metabolism. * **Enzyme Defect:** The most common genetic cause of elevated homocysteine is a mutation in **MTHFR (Methylenetetrahydrofolate reductase)**. * **Triad of Risk:** Elevated homocysteine is a "triple threat" risk factor for **Dementia, Cardiovascular Disease (MI), and Stroke.**

Question 314: What is true of vitamin D resistant rickets?

A. X-linked recessive
B. No end organ resistance to 1, 25(OH)2D3
C. Defect in proximal tubular reabsorption (Correct Answer)
D. Hyperphosphatemia and high 1, 25(OH)2D3

Explanation: **Explanation:** **Vitamin D Resistant Rickets (VDRR)**, also known as **X-linked Hypophosphatemic Rickets (XLH)**, is the most common form of heritable rickets. 1. **Why Option C is Correct:** The primary defect lies in the **proximal renal tubules**, where there is a failure to reabsorb phosphate. This is caused by mutations in the *PHEX* gene, leading to elevated levels of **FGF-23** (a phosphaturic hormone). FGF-23 inhibits the sodium-phosphate cotransporters (NPT2a/2c) in the proximal tubule, causing profound **phosphaturia** (phosphate wasting) and subsequent hypophosphatemia, which impairs bone mineralization. 2. **Why Other Options are Incorrect:** * **Option A:** VDRR is inherited as an **X-linked Dominant** trait, not recessive. It affects both males and females, though males often present more severely. * **Option B:** While XLH does not involve direct receptor resistance, another form called *Hereditary Vitamin D-Resistant Rickets (Type II)* is characterized specifically by **end-organ resistance** due to mutations in the Vitamin D Receptor (VDR). * **Option D:** The hallmark is **Hypophosphatemia** (due to renal wasting). Furthermore, FGF-23 suppresses the 1-alpha-hydroxylase enzyme, leading to **low or inappropriately normal 1,25(OH)₂D₃** levels despite low serum phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **Triad:** Hypophosphatemia, phosphaturia, and normal serum calcium. * **Key Hormone:** Elevated **FGF-23** is the diagnostic driver. * **Treatment:** Unlike nutritional rickets, it does not respond to Vitamin D alone; it requires **oral phosphate supplements** and **calcitriol** (active Vit D). * **New Therapy:** **Burosumab** (a monoclonal antibody against FGF-23) is now a specific treatment.

Question 315: The presence of a positive Ferric chloride test in urine is attributed to which of the following compounds?

A. Phenylalanine
B. Phenylacetate
C. Phenylpyruvate (Correct Answer)
D. Phenylaspartate

Explanation: ### Explanation **Correct Answer: C. Phenylpyruvate** The **Ferric chloride (FeCl₃) test** is a classic biochemical screening tool used to detect specific metabolites in urine. In patients with **Phenylketonuria (PKU)**, a deficiency of the enzyme *Phenylalanine Hydroxylase* leads to the accumulation of Phenylalanine. When the primary metabolic pathway is blocked, Phenylalanine undergoes alternative metabolism via transamination to form **Phenylpyruvate** (a phenylketone), Phenylacetate, and Phenyllactate. The Ferric chloride test specifically reacts with the **enol group of Phenylpyruvate**, producing a characteristic **transient blue-green color**. Therefore, while Phenylalanine is the primary amino acid that increases in the blood, it is its keto-acid derivative, Phenylpyruvate, that is responsible for the positive urine test. **Analysis of Incorrect Options:** * **A. Phenylalanine:** This is the substrate that accumulates in the blood, but it does not react with Ferric chloride to produce a color change. * **B. Phenylacetate:** This metabolite is responsible for the characteristic **"mousy" or "musty" odor** of urine in PKU patients, but it does not give a positive Ferric chloride test. * **D. Phenylaspartate:** This is not a significant metabolite in the phenylalanine degradation pathway. **High-Yield Clinical Pearls for NEET-PG:** * **PKU Presentation:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Guthrie Test:** A bacterial inhibition assay (using *B. subtilis*) used for neonatal screening of PKU; it detects Phenylalanine, not Phenylpyruvate. * **Other FeCl₃ Test Results:** * **Alkaptonuria:** Transient deep blue/green. * **Maple Syrup Urine Disease (MSUD):** Navy blue. * **Tyrosinemia:** Pale green. * **Salicylates/Phenothiazines:** Purple/Violet.

Question 316: What is the enzyme deficiency in albinism?

A. Tyrosinase (Correct Answer)
B. Tyrosine hydroxylase
C. Phenylalanine hydroxylase
D. Homogentisate oxidase

Explanation: **Explanation:** **1. Why Tyrosinase is the Correct Answer:** Albinism (specifically Oculocutaneous Albinism Type 1) is caused by a deficiency of the copper-containing enzyme **Tyrosinase**. In the melanocytes, Tyrosinase is the rate-limiting enzyme responsible for the first two steps of melanin synthesis: the hydroxylation of **Tyrosine to DOPA** (Dihydroxyphenylalanine) and the subsequent oxidation of **DOPA to Dopaquinone**. A lack of this enzyme results in a total or partial absence of melanin pigment in the skin, hair, and eyes. **2. Analysis of Incorrect Options:** * **B. Tyrosine hydroxylase:** While it also converts Tyrosine to DOPA, this enzyme is found in the central nervous system and adrenal medulla. It is the rate-limiting step for **catecholamine synthesis** (Dopamine, Epinephrine, Norepinephrine), not melanin. * **C. Phenylalanine hydroxylase:** Deficiency of this enzyme leads to **Phenylketonuria (PKU)**. While PKU patients may have fair skin due to secondary tyrosine deficiency, it is not the primary cause of albinism. * **D. Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine and ochronosis (pigmentation of connective tissue), which is the opposite of albinism. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most forms of Albinism are **Autosomal Recessive**. * **Clinical Features:** Photophobia, nystagmus, decreased visual acuity, and a high risk of **Squamous Cell Carcinoma** of the skin. * **Biochemical Link:** Tyrosine is the common precursor for Melanin, Catecholamines, and Thyroid hormones (T3/T4). * **Key Test:** The **Hair bulb incubation test** can help differentiate between tyrosinase-positive and tyrosinase-negative albinism.

Question 317: Which of the following statements is FALSE regarding Crigler-Najjar syndrome type II?

A. It is inherited in an autosomal dominant pattern.
B. Kernicterus is frequently present. (Correct Answer)
C. Affected children may survive into adolescence.
D. It causes unconjugated hyperbilirubinemia.

Explanation: **Explanation:** Crigler-Najjar Syndrome (CNS) is a rare disorder of bilirubin metabolism caused by a deficiency in the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. Understanding the distinction between Type I and Type II is high-yield for NEET-PG. **1. Why Option B is the correct (FALSE) statement:** In **Crigler-Najjar Type II (Arias Syndrome)**, there is a partial deficiency of UGT1A1 (enzyme activity is <10% of normal). While bilirubin levels are elevated (typically 6–20 mg/dL), they are generally not high enough to cross the blood-brain barrier in significant amounts. Therefore, **kernicterus is rare** in Type II. In contrast, Type I involves a total absence of the enzyme, leading to massive hyperbilirubinemia (>20 mg/dL) where kernicterus is frequent and often fatal. **2. Analysis of other options:** * **Option A (Inheritance):** Unlike Type I (Autosomal Recessive), Type II is most commonly inherited in an **Autosomal Dominant** pattern with variable penetrance. * **Option C (Survival):** Because the hyperbilirubinemia is less severe and responds to treatment, affected individuals usually survive into **adolescence and adulthood**. * **Option D (Type of Bilirubin):** Since the defect lies in the conjugation process in the liver, it characteristically causes **unconjugated (indirect) hyperbilirubinemia**. **Clinical Pearls for NEET-PG:** * **Phenobarbital Test:** This is the gold standard for differentiating the two types. Phenobarbital induces the remaining UGT1A1 enzyme in **Type II**, resulting in a >25% reduction in serum bilirubin. It has **no effect** on Type I. * **Treatment:** Type II is managed with Phenobarbital; Type I requires phototherapy and eventually a liver transplant. * **Gilbert Syndrome:** The mildest form of UGT1A1 deficiency (reduced promoter expression), presenting with mild jaundice during stress.

Question 318: An infant presented with a history of vomiting and poor feeding. A musty odor is present in the baby's sweat and urine. The Guthrie test was positive. Which of the following is NOT true regarding this condition?

A. Phenylacetate is positive in urine (Correct Answer)
B. Tandem mass spectrometry is the gold standard test
C. There is a defect in the phenylalanine hydroxylase enzyme
D. Mental retardation is absent

Explanation: The clinical presentation of vomiting, poor feeding, a **musty (mousy) odor**, and a positive **Guthrie test** (a bacterial inhibition assay) confirms a diagnosis of **Phenylketonuria (PKU)**. ### **Analysis of Options** * **Option D (Mental Retardation is absent):** This is the **incorrect statement** (and thus the correct answer for a "NOT true" question). Untreated PKU leads to severe intellectual disability due to the accumulation of phenylalanine, which interferes with amino acid transport across the blood-brain barrier and inhibits myelin formation. * **Option A (Phenylacetate is positive in urine):** This is a **true** statement. In PKU, phenylalanine is diverted into alternative pathways, forming phenylpyruvate, phenyllactate, and **phenylacetate**. Phenylacetate is specifically responsible for the characteristic "mousy" odor. * **Option B (Tandem mass spectrometry is the gold standard):** This is **true**. While the Guthrie test was historically used for screening, Tandem Mass Spectrometry (TMS) is now the gold standard for neonatal screening due to its high sensitivity and specificity. * **Option C (Defect in phenylalanine hydroxylase):** This is **true**. Classic PKU (Type I) is caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**, which converts phenylalanine to tyrosine. ### **Clinical Pearls for NEET-PG** * **Inheritance:** Autosomal Recessive. * **Biochemical Hallmark:** Hyperphenylalaninemia and low Tyrosine (Tyrosine becomes an **essential** amino acid in these patients). * **Hypopigmentation:** Patients often have fair skin and blue eyes because phenylalanine inhibits tyrosinase, reducing melanin synthesis. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus.

Question 319: A 6-month-old boy presented with failure to thrive. Laboratory investigations revealed hypoglycemia and elevated urinary glutamine and uracil levels. Gastric tube feeding was poorly tolerated. The child became comatose, and after parenteral dextrose administration, recovered within 24 hours. Which of the following enzymes is defective?

A. Argininosuccinate synthase
B. Carbamoyl phosphate synthase I
C. Argininosuccinate lyase
D. Ornithine transcarbamoylase (Correct Answer)

Explanation: ### Explanation The clinical presentation of hyperammonemia (coma), failure to thrive, and specific biochemical markers points toward a **Urea Cycle Disorder (UCD)**. **1. Why Ornithine Transcarbamoylase (OTC) is correct:** OTC deficiency is the most common urea cycle defect. When OTC is defective, **Carbamoyl Phosphate (CP)** cannot combine with ornithine to form citrulline. Consequently, excess CP leaks out of the mitochondria into the cytosol, where it enters the pyrimidine synthesis pathway. This leads to an overproduction of **Orotic acid and Uracil**, which are excreted in the urine. The elevation of **Glutamine** is a hallmark of hyperammonemia, as ammonia is detoxified into glutamine in the brain and blood. **2. Why the other options are incorrect:** * **CPS-I Deficiency (Option B):** This also causes severe hyperammonemia, but since the defect occurs *before* the formation of Carbamoyl Phosphate, there is **no elevation** of urinary orotic acid or uracil. * **Argininosuccinate Synthase (Option A) & Lyase (Option C):** These are "downstream" defects (Citrullinemia and Argininosuccinic aciduria). While they cause hyperammonemia, they typically present with significantly elevated levels of citrulline or argininosuccinate, rather than the specific uracil/orotic acid pattern seen in OTC deficiency. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** OTC deficiency is the **only** Urea Cycle Disorder that is **X-linked Recessive**; all others are Autosomal Recessive. * **Differential Diagnosis:** To distinguish between CPS-I and OTC deficiency, look at **Orotic Acid/Uracil**: High = OTC; Low/Normal = CPS-I. * **Management:** Acute treatment involves stopping protein intake, giving IV glucose (to prevent catabolism), and using ammonia scavengers like Sodium Benzoate.

Question 320: What is the most common enzyme deficiency leading to porphyria in humans?

A. PBG deaminase
B. Uroporphyrinogen decarboxylase (Correct Answer)
C. Ferrochelatase
D. Coproporphyrinogen oxidase

Explanation: **Explanation:** The correct answer is **Uroporphyrinogen decarboxylase (UROD)**. Deficiency of UROD leads to **Porphyria Cutanea Tarda (PCT)**, which is statistically the **most common porphyria** worldwide. PCT is unique because it can be either inherited (Type II) or, more commonly, acquired (Type I) due to factors like alcohol consumption, Hepatitis C, or iron overload. The enzyme UROD is responsible for converting uroporphyrinogen III to coproporphyrinogen III in the heme synthesis pathway. **Analysis of Incorrect Options:** * **A. PBG deaminase:** Deficiency causes **Acute Intermittent Porphyria (AIP)**. While AIP is the most common *acute* (neurological) porphyria, it is less common overall than PCT. * **C. Ferrochelatase:** Deficiency leads to **Erythropoietic Protoporphyria (EPP)**. This is the most common porphyria in children but not the most common in the general human population. * **D. Coproporphyrinogen oxidase:** Deficiency causes **Hereditary Coproporphyria (HCP)**, which is a rare autosomal dominant acute porphyria. **High-Yield Clinical Pearls for NEET-PG:** * **Porphyria Cutanea Tarda (PCT):** Characterized by **photosensitivity** and blistering/vesicles on sun-exposed skin (dorsum of hands). It is associated with **Hepatitis C** and "tea-colored" urine. * **Acute Intermittent Porphyria (AIP):** Presents with the "5 Ps": **P**ainful abdomen, **P**ort-wine urine, **P**olyneuropathy, **P**sychological disturbances, and **P**recipitated by drugs (e.g., Barbiturates, Cytochrome P450 inducers). * **Key Enzyme Rule:** Deficiencies *after* the formation of uroporphyrinogen lead to photosensitivity; deficiencies *before* that step (like AIP) typically do not.

Practice by Chapter

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