Genetic Disorders and Biochemical Pathology Practice Questions

Q: An infant presents with a history of seizures and skin rashes. Investigations reveal metabolic acidosis and increased blood ketone levels. Which of the following conditions is this child most likely suffering from?

Multiple carboxylase deficiency. **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is the correct diagnosis because it uniquely accounts for the triad of **seizures, skin rashes (alopecia/dermatitis), and metabolic acidosis with ketosis.** MCD occurs due to a defect in either **Holocarboxylase synthetase** (neonatal onset) or **Biotinidase** (late-onset). Biotin is a mandatory cofactor for four essential enzymes: 1. **Pyruvate carboxylase:** Deficiency leads to lactic acidosis. 2. **Propionyl-CoA carboxylase:** Deficiency leads to propionic acidemia and ketosis. 3. **Acetyl-CoA carboxylase:** Impairs fatty acid synthesis. 4. **3-Methylcrotonyl-CoA carboxylase:** Leads to characteristic organic aciduria. The skin rash and neurological symptoms are hallmark features of biotin-related disorders. **Why other options are incorrect:** * **Propionic Acidemia:** While it causes metabolic acidosis and ketosis, it typically lacks the characteristic **skin rashes** seen in MCD. * **Urea Cycle Disorders:** These typically present with **hyperammonemia and respiratory alkalosis**, not metabolic acidosis or ketosis. * **Phenylketonuria (PKU):** Presents with intellectual disability and a "mousy odor," but does not cause acute metabolic acidosis or ketosis. **High-Yield Clinical Pearls for NEET-PG:** * **Biotinidase deficiency** is often called the "late-onset" form of MCD and is treatable with oral biotin supplementation. * **The "Skin + Acidosis" Clue:** In any biochemistry question, the combination of organic acidemia (acidosis/ketosis) plus dermatological findings (rash/alopecia) should immediately point toward **Biotin/Multiple Carboxylase Deficiency.** * **Key Enzyme:** Biotinidase is required to release biotin from biocytin (derived from dietary proteins).

Q: In all the following metabolic disorders, clinical features are seen due to accumulation of toxic compounds EXCEPT?

Mitochondrial disorders. ### Explanation The fundamental distinction in metabolic pathology lies between **"Intoxication-type"** disorders and **"Energy-deficiency"** disorders. **1. Why Mitochondrial Disorders is the Correct Answer:** Mitochondrial disorders (e.g., MELAS, Leigh syndrome) primarily result from a **failure in energy production (ATP)** rather than the accumulation of a specific upstream toxic metabolite. The clinical features—such as encephalopathy, myopathy, and lactic acidosis—arise because tissues with high metabolic demands (brain, muscle, heart) cannot meet their energy requirements due to defects in the respiratory chain or oxidative phosphorylation. **2. Why the Other Options are Incorrect:** * **Phenylketonuria (PKU):** Caused by a deficiency of phenylalanine hydroxylase. Clinical features (intellectual disability, musty odor) are due to the **toxic accumulation of phenylalanine** and its metabolites (phenylpyruvate). * **Maple Syrup Urine Disease (MSUD):** Caused by a defect in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. Symptoms (seizures, encephalopathy) are due to the **toxic buildup of Leucine, Isoleucine, and Valine.** * **Urea Cycle Defects:** Defects in enzymes like Ornithine Transcarbamylase (OTC) lead to the **toxic accumulation of Ammonia**, which is highly neurotoxic and causes cerebral edema. ### NEET-PG High-Yield Pearls: * **Intoxication Disorders:** Characterized by a "symptom-free interval" after birth followed by acute crisis (vomiting, coma) triggered by protein intake. (Includes: Organic acidemias, Urea cycle defects, Galactosemia). * **Energy Deficiency Disorders:** Often present with hypoglycemia, hypotonia, and cardiomyopathy. (Includes: Mitochondrial disorders, Fatty acid oxidation defects, Glycogen storage diseases). * **Mnemonic:** In **M**itochondrial diseases, the **M**achinery (ATP production) is broken; in others, the **P**ipe is clogged (Toxic buildup).

Q: Which of the following is NOT an X-linked disease?

Wilson's disease. **Explanation:** The correct answer is **Wilson’s disease** because it follows an **Autosomal Recessive** inheritance pattern, not X-linked. **1. Why Wilson’s Disease is the correct answer:** Wilson’s disease is caused by mutations in the **ATP7B gene** located on **Chromosome 13**. This defect leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and cornea (Kayser-Fleischer rings). Since the gene is on an autosome, it affects males and females equally. **2. Why the other options are incorrect:** * **Androgen Insensitivity Syndrome (AIS):** This is an **X-linked recessive** condition caused by mutations in the androgen receptor gene on the X chromosome. Affected individuals are genotypically male (46, XY) but phenotypically female. * **Duchenne Muscular Dystrophy (DMD):** This is a classic **X-linked recessive** disorder caused by a mutation in the *Dystrophin* gene (the largest known human gene), located on the X chromosome. * **Hemophilia:** Both Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) are inherited in an **X-linked recessive** manner. **High-Yield Clinical Pearls for NEET-PG:** * **Wilson vs. Menkes:** While Wilson’s is Autosomal Recessive (ATP7B), **Menkes disease** (kinky hair syndrome) is **X-linked Recessive** (ATP7A). * **Common X-linked Recessive Mnemonic:** "**G**o **C**herish **H**is **D**iamonds" (**G**6PD deficiency, **C**olor blindness, **H**emophilia/Hunter syndrome, **D**MD). * **Wilson’s Diagnosis:** Look for low serum ceruloplasmin, increased urinary copper, and the "Giant Panda" sign on brain MRI.

Q: An albino girl is married to a normal boy. What are the chances of their having an affected child and what are the chances of their children being carriers?

None affected, all carriers. ### Explanation **1. Understanding the Correct Answer (Option A)** Albinism (specifically Oculocutaneous Albinism) is a classic **Autosomal Recessive (AR)** disorder. For an individual to be affected, they must possess two mutant alleles (genotype **aa**). A "normal" individual, unless specified as a carrier, is assumed to be homozygous dominant (genotype **AA**) in medical genetics problems. * **The Cross:** Mother (aa) x Father (AA) * **Punnett Square:** All offspring will have the genotype **Aa**. * **Phenotype:** Since they possess one dominant allele (A), none will manifest the disease (None affected). * **Genotype:** Since they all carry one recessive allele (a), 100% will be obligate heterozygotes (All carriers). **2. Analysis of Incorrect Options** * **Option B (All normal):** While phenotypically true, it is incomplete as it ignores the carrier status, which is a critical genetic implication. * **Option C (50% carriers):** This would only occur if the father was a carrier (Aa) and the mother was homozygous normal (AA). * **Option D (50% affected, 50% carriers):** This is the result of a **Test Cross** (aa x Aa). It only occurs if the "normal" father was actually a carrier (pseudodominance). **3. NEET-PG Clinical Pearls** * **Biochemical Defect:** Most commonly due to a deficiency of the enzyme **Tyrosinase**, which converts Tyrosine to DOPA and Melanin. * **Inheritance Pattern:** Always assume Autosomal Recessive for Albinism unless Ocular Albinism (X-linked) is specifically mentioned. * **Key Association:** Albinism is a feature of **Chédiak-Higashi syndrome** (look for giant cytoplasmic granules in neutrophils and recurrent infections). * **Risk Calculation:** In AR disorders, if both parents are carriers (Aa x Aa), the risk of an affected child is 25%, and the risk of a healthy child being a carrier is 2/3 (66%).

Q: Which statement is false regarding Gilbert syndrome?

Mild conjugated hyperbilirubinemia. **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.

Q: What is true of vitamin D resistant rickets?

Defect in proximal tubular reabsorption. **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.

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

Kernicterus is frequently present.. **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 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.

Q: Which genetic disease is exclusively transmitted to females?

Hunter's disease. ### Explanation **Correct Answer: B. Hunter’s disease** The question focuses on the mode of inheritance of Mucopolysaccharidoses (MPS). * **Hunter’s disease (MPS II)** is unique among the mucopolysaccharidoses because it is inherited in an **X-linked recessive** pattern. * In X-linked recessive disorders, a carrier mother has a 50% chance of transmitting the defective gene to her offspring. While males are primarily affected (as they are hemizygous), **females can be carriers** and, in rare cases of skewed X-inactivation (Lyonization), can manifest the disease. * *Note on Question Phrasing:* In medical entrance exams, "transmitted to females" often refers to the carrier status or the genetic transmission through the maternal line, distinguishing it from Autosomal Recessive traits where both parents must contribute a gene. **Why the other options are incorrect:** * **A & C (Hurler’s and Scheie’s disease):** Both are subtypes of **MPS I** (deficiency of α-L-iduronidase). All forms of MPS I are inherited in an **Autosomal Recessive** manner, affecting males and females equally. * **D (Fabry’s disease):** While Fabry’s is also X-linked recessive, it is a **Sphingolipidosis**, not a Mucopolysaccharidosis. In the context of comparing MPS types (the primary theme of the options), Hunter’s is the classic "exception to the rule." **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic:** "The **Hunter** needs **X**-ray vision to see the **Target** (No corneal clouding)." * **X:** X-linked recessive inheritance. * **Target:** Presence of "Target-like" skin lesions (Pebbling). 2. **Key Distinction:** Unlike Hurler’s (MPS I), Hunter’s (MPS II) presents **without corneal clouding**. 3. **Enzyme Deficiency:** Hunter’s is caused by a deficiency of **Iduronate-2-sulfatase**. 4. **Accumulated Substances:** Dermatan sulfate and Heparan sulfate.

Question 1

An infant presents with a history of seizures and skin rashes. Investigations reveal metabolic acidosis and increased blood ketone levels. Which of the following conditions is this child most likely suffering from?

Accepted Answer

Multiple carboxylase deficiency

Answer

Propionic acidemia

Answer

Urea cycle disorder

Answer

Phenylketonuria

Question 2

In all the following metabolic disorders, clinical features are seen due to accumulation of toxic compounds EXCEPT?

Accepted Answer

Mitochondrial disorders

Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Urea cycle defects

Question 3

Which of the following is NOT an X-linked disease?

Accepted Answer

Wilson's disease

Answer

Androgen insensitivity syndrome

Answer

Duchenne muscular dystrophy

Answer

Hemophilia

Question 4

A 29-year-old man has had angina for the past year with a family history of cardiovascular disease. His blood pressure is 120/80 mm Hg, total serum cholesterol is 185 mg/dL, and glucose is 85 mg/dL. A mutation involving a gene encoding for which of the following is most likely present in this man's family?

Accepted Answer

Apolipoprotein

Answer

Angiotensin

Answer

Endothelin

Answer

Factor VIII

Question 5

An albino girl is married to a normal boy. What are the chances of their having an affected child and what are the chances of their children being carriers?

Accepted Answer

None affected, all carriers

Answer

All normal

Answer

50% carriers

Answer

50% affected, 50% carriers

Question 6

Which statement is false regarding Gilbert syndrome?

Accepted Answer

Mild conjugated hyperbilirubinemia

Answer

Male predominant

Answer

UGT activity is 10-30% of normal

Answer

Histology normal

Question 7

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

Accepted Answer

Homocysteine

Answer

Cysteine

Answer

Taurine

Answer

Methionine

Question 8

What is true of vitamin D resistant rickets?

Accepted Answer

Defect in proximal tubular reabsorption

Answer

X-linked recessive

Answer

No end organ resistance to 1, 25(OH)2D3

Answer

Hyperphosphatemia and high 1, 25(OH)2D3

Question 9

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

Accepted Answer

Kernicterus is frequently present.

Answer

It is inherited in an autosomal dominant pattern.

Answer

Affected children may survive into adolescence.

Answer

It causes unconjugated hyperbilirubinemia.

Question 10

Which genetic disease is exclusively transmitted to females?

Accepted Answer

Hunter's disease

Answer

Hurler's disease

Answer

Scheie's disease

Answer

Fabry's disease

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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