Genetic Disorders and Biochemical Pathology — MCQs

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

Question 501: All of the following are mitochondrial disorders EXCEPT?

A. MELAS
B. Kearns Sayre syndrome
C. NARP syndrome
D. Incontinentia pigmenti (Correct Answer)

Explanation: **Explanation:** The correct answer is **Incontinentia pigmenti (D)**. **1. Why Incontinentia pigmenti is the correct answer:** Incontinentia pigmenti (Bloch-Sulzberger syndrome) is an **X-linked dominant** neurocutaneous disorder, not a mitochondrial disorder. It is caused by a mutation in the **IKBKG (NEMO) gene**, which is essential for the activation of NF-kappaB, a protein that protects cells against apoptosis. It typically presents with characteristic skin lesions (vesicular, verrucous, and hyperpigmented stages) and is usually lethal in males. **2. Why the other options are incorrect (Mitochondrial Disorders):** Mitochondrial disorders are characterized by **maternal inheritance** (mitochondrial DNA is inherited only from the mother) and **heteroplasmy**. They primarily affect high-energy demanding tissues like the brain and muscles. * **MELAS:** Stands for Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes. It is most commonly due to a mutation in the tRNA leucine gene. * **Kearns-Sayre Syndrome (KSS):** Characterized by the triad of progressive external ophthalmoplegia (PEO), pigmentary retinopathy, and onset before age 20. It involves large-scale mtDNA deletions. * **NARP Syndrome:** Stands for Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa. It is caused by mutations in the ATP6 gene of mtDNA. **Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** If a mother is affected, all children are at risk; if a father is affected, zero children are at risk. * **Ragged Red Fibers:** A hallmark histological finding in mitochondrial myopathies (seen on Gomori trichrome stain). * **Leber Hereditary Optic Neuropathy (LHON):** Another high-yield mitochondrial disorder causing painless bilateral vision loss.

Question 502: Which one of the following disorders is autosomal recessive?

A. Homocystinuria (Correct Answer)
B. G6PD deficiency
C. Myotonic dystrophy
D. Otospongiosis

Explanation: **Explanation:** **Correct Answer: A. Homocystinuria** Homocystinuria is an **Autosomal Recessive (AR)** disorder, most commonly caused by a deficiency of the enzyme **Cystathionine beta-synthase (CBS)**. As a general rule for NEET-PG, almost all **Inborn Errors of Metabolism (IEMs)** follow an autosomal recessive inheritance pattern (exceptions include Hunter syndrome and Fabry disease). In Homocystinuria, the accumulation of homocysteine leads to a classic tetrad of clinical features: ectopia lentis (downward dislocation), marfanoid habitus, intellectual disability, and high risk of thromboembolism. **Analysis of Incorrect Options:** * **B. G6PD deficiency:** This is an **X-linked Recessive (XLR)** disorder. It is the most common red cell enzyme deficiency worldwide, leading to episodic hemolytic anemia triggered by oxidative stress (e.g., fava beans, primaquine, or infections). * **C. Myotonic dystrophy:** This is an **Autosomal Dominant (AD)** disorder. It is characterized by "anticipation" due to trinucleotide repeat expansion (CTG repeats in the DMPK gene). * **D. Otospongiosis (Otosclerosis):** This is typically inherited in an **Autosomal Dominant** pattern with variable penetrance. It involves abnormal bone remodeling in the middle ear, leading to conductive hearing loss. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for AR disorders:** "All enzymes are AR" (except those that are X-linked). * **Homocystinuria vs. Marfan Syndrome:** Both present with marfanoid habitus, but Homocystinuria has **downward** lens dislocation and intellectual disability, whereas Marfan is AD and has **upward** lens dislocation. * **Treatment:** A subset of Homocystinuria patients responds dramatically to high doses of **Vitamin B6 (Pyridoxine)**, which acts as a cofactor for the CBS enzyme.

Question 503: Which of the following is found in urine in patients with Alkaptonuria?

A. Phenylalanine
B. Ornithine
C. Cystine
D. Homogentisic acid (Correct Answer)

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. It is caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, which is responsible for converting **Homogentisic acid (HGA)** into maleylacetoacetic acid. Due to this block, HGA accumulates in the blood and is excreted in large amounts in the urine. 1. **Why Homogentisic acid is correct:** When urine containing HGA is exposed to air or an alkaline environment, it undergoes oxidation and polymerization to form a brownish-black pigment (alkapton). This leads to the classic clinical sign: **urine that turns black on standing.** 2. **Why other options are incorrect:** * **Phenylalanine:** Elevated in Phenylketonuria (PKU) due to Phenylalanine hydroxylase deficiency. * **Ornithine & Cystine:** These, along with Lysine and Arginine (COLA), are found in the urine of patients with **Cystinuria**, a defect in renal tubular reabsorption of dibasic amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues (ear cartilage, sclera, and heart valves). * **Ochronotic Arthritis:** Large joint arthritis (especially the spine and hips) is a common late-stage complication. * **Diagnosis:** Confirmed by detecting HGA in urine using **Ferric Chloride test** (transient deep blue color) or Silver Nitrate test. * **Management:** Low protein diet (restricted Tyrosine/Phenylalanine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing HGA production.

Question 504: In porphyria cutanea tarda, what type of porphyrin is excreted in the urine?

A. Coproporphyrin - I
B. Uroporphyrin - I
C. Uroporphyrin - II
D. Type I and III uroporphyrins (Correct Answer)

Explanation: **Explanation:** **Porphyria Cutanea Tarda (PCT)** is the most common type of porphyria. It results from a deficiency of the enzyme **Uroporphyrinogen Decarboxylase (UROD)**. 1. **Why the correct answer is right:** Under normal physiological conditions, UROD converts Uroporphyrinogen III into Coproporphyrinogen III. When UROD is deficient, its substrate—**Uroporphyrinogen III**—accumulates in the liver. This excess uroporphyrinogen is non-enzymatically oxidized into **Uroporphyrin III**, which is then excreted in the urine. Additionally, due to the metabolic shunt caused by the block, there is an increased production and excretion of **Uroporphyrin I**. Therefore, the hallmark urinary finding in PCT is a significant elevation of both **Type I and III uroporphyrins**. 2. **Why incorrect options are wrong:** * **Option A:** Coproporphyrin-I is typically elevated in Dubin-Johnson syndrome or certain secondary porphyrinurias, but it is not the primary diagnostic marker for PCT. * **Option B & C:** While Uroporphyrin-I is elevated, it does not occur in isolation. Uroporphyrin-II is not a naturally occurring isomer in human heme synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by **photosensitivity**, skin fragility, and blistering (vesicles/bullae) on sun-exposed areas (back of hands). * **Risk Factors:** Often triggered by alcohol consumption, iron overload (hemochromatosis), Hepatitis C, or estrogen use. * **Diagnosis:** Urine shows "tea-colored" or "port-wine" appearance and exhibits **pink-red fluorescence** under Wood’s lamp. * **Treatment:** Phlebotomy (to reduce iron) or low-dose chloroquine.

Question 505: A 14-year-old female on strenuous exercise presented with muscle pains and voiding red-colored urine. What is the most likely diagnosis?

A. Glycolytic pathway defect (Correct Answer)
B. Carnitine acyltransferase deficiency
C. Mitochondrial deficiency myopathy
D. Hypokalemic periodic paralysis

Explanation: ### Explanation The clinical presentation of **exercise-induced muscle pain** and **myoglobinuria** (red-colored urine) in a teenager points toward a metabolic myopathy. **1. Why Glycolytic Pathway Defect is Correct:** The most likely diagnosis is **McArdle Disease (GSD Type V)**, a defect in muscle glycogen phosphorylase, or a similar glycolytic defect (e.g., Tarui disease). In these conditions, the muscle cannot break down glycogen into glucose-6-phosphate to fuel **anaerobic glycolysis**. During **strenuous/intense exercise**, the body relies heavily on rapid ATP production via glycolysis. A block here leads to an energy crisis, muscle fiber breakdown (rhabdomyolysis), and subsequent myoglobinuria. A classic sign is the "second wind" phenomenon. **2. Why Other Options are Incorrect:** * **Carnitine Acyltransferase (CPT II) Deficiency:** While this also causes exercise-induced myoglobinuria, it typically occurs after **prolonged, low-intensity endurance exercise** or fasting, as it affects fatty acid oxidation rather than immediate glycolytic energy. * **Mitochondrial Deficiency Myopathy:** These usually present with progressive muscle weakness, exercise intolerance, and **lactic acidosis**, but myoglobinuria is less common. * **Hypokalemic Periodic Paralysis:** This presents as episodes of flaccid muscle paralysis, often triggered by high-carbohydrate meals or rest after exercise, but it does **not** cause myoglobinuria or muscle pain. **Clinical Pearls for NEET-PG:** * **Ischemic Forearm Exercise Test:** Failure of blood lactate to rise after exercise is diagnostic for McArdle disease. * **Myoglobinuria vs. Hematuria:** In myoglobinuria, the urine dipstick is positive for blood, but microscopy shows **no RBCs**. * **Key Enzyme:** Muscle Glycogen Phosphorylase (absent in McArdle).

Question 506: Which substance gets accumulated in abnormal amounts in Alkaptonuria?

A. Acetoacetate
B. Oxalate
C. Homogentisate (Correct Answer)
D. Phenylacetic acid

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. It is caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, which is responsible for converting homogentisic acid (homogentisate) into maleylacetoacetate. Due to this metabolic block, **homogentisate** accumulates in the blood and is excreted in large amounts in the urine. **Why the other options are incorrect:** * **Acetoacetate:** This is a ketone body and a downstream product of tyrosine metabolism. In Alkaptonuria, the pathway is blocked upstream, preventing its formation from this specific pathway. * **Oxalate:** Accumulation of oxalate is characteristic of Primary Hyperoxaluria, not tyrosine metabolism disorders. * **Phenylacetic acid:** This metabolite accumulates in Phenylketonuria (PKU) due to the alternative pathway of phenylalanine metabolism when phenylalanine hydroxylase is deficient. **High-Yield Clinical Pearls for NEET-PG:** 1. **Triad of Alkaptonuria:** * **Homogentisic aciduria:** Urine turns black upon standing or alkalinization (due to oxidation of homogentisate to benzoquinone acetate). * **Ochronosis:** Bluish-black pigmentation of connective tissues (ear cartilage, sclera) due to polymer deposition. * **Ochronotic Arthritis:** Large joint arthritis caused by pigment deposition in the cartilage. 2. **Diagnosis:** Confirmed by detecting homogentisic acid in urine using **Ferric Chloride test** (transient deep blue color) or Gas Chromatography-Mass Spectrometry (GC-MS). 3. **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing homogentisate production.

Question 507: A 10-month-old male child presents with vomiting, lethargy, and severe jaundice, which began when weaning was initiated with fruit juice. Laboratory investigations reveal prolonged clotting time, hypoalbuminemia, and elevated bilirubin and transaminase levels. What is the diagnosis?

A. Von Gierke's disease
B. Hereditary fructose intolerance (Correct Answer)
C. Benign fructosuria
D. Lactose intolerance

Explanation: ### Explanation **Correct Answer: B. Hereditary Fructose Intolerance (HFI)** The clinical presentation is classic for **Hereditary Fructose Intolerance**, an autosomal recessive disorder caused by a deficiency of **Aldolase B**. * **Mechanism:** When weaning begins (introduction of fruit juices or honey), fructose is ingested. Due to Aldolase B deficiency, **Fructose-1-Phosphate (F1P)** accumulates in hepatocytes. * **Pathology:** The accumulation of F1P "traps" intracellular inorganic phosphate ($P_i$). This depletion of $P_i$ inhibits glycogenolysis and gluconeogenesis, leading to severe postprandial hypoglycemia. Furthermore, the high levels of F1P are toxic to the liver, causing hepatocellular damage, jaundice, and impaired synthetic function (hypoalbuminemia and prolonged clotting time). --- ### Why the other options are incorrect: * **A. Von Gierke's Disease (GSD Type I):** While it presents with hypoglycemia and hepatomegaly, it is caused by Glucose-6-Phosphatase deficiency. It typically presents earlier (3–4 months) and is characterized by lactic acidosis, hyperuricemia, and hyperlipidemia, rather than acute liver failure triggered by fruit juice. * **C. Benign Fructosuria:** Caused by **Fructokinase** deficiency. It is an asymptomatic condition where fructose is excreted in the urine. There is no liver damage or hypoglycemia because F1P does not accumulate. * **D. Lactose Intolerance:** This presents with osmotic diarrhea, bloating, and abdominal cramps following milk ingestion. It does not cause jaundice, liver failure, or hypoglycemia. --- ### NEET-PG High-Yield Pearls: * **The "Weaning" Trigger:** Always suspect HFI or Galactosemia in infants. If symptoms start with **milk** (lactose), think Galactosemia; if they start with **fruit/honey** (fructose), think HFI. * **Enzyme Deficiency:** Aldolase B (HFI) vs. Fructokinase (Essential Fructosuria). * **Reducing Sugars:** Both HFI and Galactosemia will show a positive Benedict's test (reducing sugar in urine) but a negative glucose oxidase dipstick test. * **Management:** Strict exclusion of fructose, sucrose, and sorbitol from the diet.

Question 508: A 5-day-old child presents with intractable seizures and generalized rashes. Blood examination reveals hyperammonemia and lactic acidosis. What is the probable diagnosis?

A. Organic aciduria (Correct Answer)
B. Mitochondrial encephalopathy with lactic aciduria
C. Phenylketonuria
D. Urea cycle enzyme deficiency

Explanation: **Explanation:** The clinical presentation of a neonate with **intractable seizures, generalized rashes, hyperammonemia, and lactic acidosis** is classic for **Organic Acidurias** (e.g., Propionic acidemia, Methylmalonic acidemia, or Multiple carboxylase deficiency). **1. Why Organic Aciduria is correct:** Organic acidurias are caused by defects in the catabolism of branched-chain amino acids. The accumulation of organic acids leads to **lactic acidosis** (due to inhibition of the pyruvate dehydrogenase complex) and **hyperammonemia** (due to inhibition of N-acetylglutamate synthase, which is essential for the urea cycle). The presence of **rashes** (often eczematous or seborrheic) is a specific high-yield clue, particularly in Biotinidase deficiency or Multiple carboxylase deficiency. **2. Why other options are incorrect:** * **Mitochondrial encephalopathy (MELAS):** While it presents with lactic acidosis and seizures, it rarely causes significant hyperammonemia or neonatal rashes. * **Phenylketonuria (PKU):** Presents later with intellectual disability and "mousy" odor; it does not cause acute neonatal hyperammonemia or lactic acidosis. * **Urea Cycle Enzyme Deficiency:** While this causes severe hyperammonemia and seizures, it typically presents with **respiratory alkalosis** (due to hyperventilation) rather than lactic acidosis. **Clinical Pearls for NEET-PG:** * **Hyperammonemia + Ketosis/Acidosis** = Organic Aciduria. * **Hyperammonemia + Alkalosis** = Urea Cycle Disorder. * **Management:** Immediate protein restriction and supplementation with specific cofactors (e.g., Biotin, B12, or Carnitine).

Question 509: Which of the following vectors can carry the largest genome?

A. Plasmids
B. BAC (Bacterial Artificial Chromosome)
C. YAC (Yeast Artificial Chromosome) (Correct Answer)
D. Cosmids

Explanation: **Explanation:** In recombinant DNA technology, the choice of a vector depends primarily on the size of the DNA insert it can accommodate. **Why YAC is the Correct Answer:** **Yeast Artificial Chromosomes (YACs)** are engineered chromosomes containing yeast telomeres, centromeres, and replication origins. They are designed to mimic natural chromosomes in *Saccharomyces cerevisiae*. Because they function like eukaryotic chromosomes, they possess the highest carrying capacity of all common vectors, typically ranging from **100 kb to 3,000 kb (3 Mb)**. This makes them indispensable for large-scale genomic mapping, such as the Human Genome Project. **Why the Other Options are Incorrect:** * **Plasmids (A):** These are small, circular extrachromosomal DNA molecules. They have the lowest capacity, generally carrying inserts of **<10 kb**. * **Cosmids (D):** These are hybrid vectors (plasmid + Lambda phage *cos* sites). They can package DNA into phage heads, allowing for inserts of **30–45 kb**. * **BAC (B):** Based on the F-plasmid of *E. coli*, BACs are more stable than YACs but have a lower capacity, typically carrying **100–300 kb**. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Capacity (Ascending):** Plasmid < Bacteriophage < Cosmid < BAC < YAC. * **Stability:** While YACs carry the most DNA, **BACs are often preferred** in labs because they are more structurally stable and easier to manipulate. * **Expression Vectors:** If the goal is to produce a human protein (e.g., Insulin) in bacteria, an **Expression Vector** (containing a promoter and ribosome binding site) must be used.

Question 510: Accumulation of glucosylceramide with enlarged liver and spleen is characteristic of which lysosomal storage disorder?

A. Gaucher disease (Correct Answer)
B. Tay-Sachs disease
C. Fabry disease
D. Niemann-Pick disease

Explanation: **Explanation:** **Gaucher disease** is the correct answer because it is characterized by a deficiency of the enzyme **glucocerebrosidase** (also known as acid β-glucosidase). This deficiency leads to the accumulation of **glucosylceramide** (glucocerebroside) within the lysosomes of macrophages. These lipid-laden macrophages, known as **Gaucher cells** (appearing as "wrinkled tissue paper" cytoplasm), primarily infiltrate the bone marrow, liver, and spleen, resulting in the hallmark clinical presentation of **hepatosplenomegaly** and bone pain/crises. **Analysis of Incorrect Options:** * **Tay-Sachs disease:** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of **GM2 ganglioside**. It presents with a cherry-red spot on the macula and neurodegeneration, but notably lacks hepatosplenomegaly. * **Fabry disease:** An X-linked recessive disorder caused by **α-galactosidase A** deficiency. The accumulating metabolite is **ceramide trihexoside**. Clinical features include angiokeratomas, peripheral neuropathy, and renal failure. * **Niemann-Pick disease:** Caused by **sphingomyelinase** deficiency, leading to **sphingolipid** accumulation. While it also presents with hepatosplenomegaly and a cherry-red spot, the key metabolite is sphingomyelin, and pathology shows "foam cells." **High-Yield Clinical Pearls for NEET-PG:** * **Most Common:** Gaucher disease is the most common lysosomal storage disorder. * **Histology:** Look for "Gaucher cells" (macrophages with fibrillary, wrinkled paper cytoplasm). * **Biochemical Marker:** Elevated serum **acid phosphatase** levels are often seen. * **Treatment:** Enzyme Replacement Therapy (ERT) with recombinant glucocerebrosidase (Imiglucerase) is the standard of care.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

Practice Questions

Hemoglobinopathies

Practice Questions

Porphyrias

Practice Questions

Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

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