Genetic Disorders and Biochemical Pathology Practice Questions

Q: All of the following are mitochondrial disorders EXCEPT?

Incontinentia pigmenti. **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.

Q: Which one of the following disorders is autosomal recessive?

Homocystinuria. **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.

Q: Which of the following is an X-linked dominant condition?

Phosphate diabetes. **Explanation:** **Phosphate diabetes** (also known as **X-linked Hypophosphatemic Rickets**) is the correct answer. It is one of the classic examples of an **X-linked dominant (XLD)** condition. It is caused by a mutation in the *PHEX* gene, leading to excess FGF-23, which causes renal phosphate wasting. In XLD inheritance, a single copy of the mutated gene on the X chromosome is sufficient to cause the disease in both males and females. **Analysis of Incorrect Options:** * **Hemophilia (A and B):** These are classic **X-linked recessive (XLR)** disorders. They primarily affect males, while females are typically asymptomatic carriers. * **Gaucher's disease:** This is a lysosomal storage disorder inherited in an **Autosomal Recessive (AR)** pattern. It is caused by a deficiency of the enzyme glucocerebrosidase. * **Cystic fibrosis:** This is the most common lethal **Autosomal Recessive (AR)** disease in Caucasians, caused by mutations in the *CFTR* gene on chromosome 7. **High-Yield Clinical Pearls for NEET-PG:** * **X-linked Dominant Mnemonics:** Remember "Alport’s, Rett Syndrome, Fragile X, and Phosphate Diabetes (Hypophosphatemic Rickets)." * **Distinguishing Feature:** In XLD, an affected father will pass the condition to **all** of his daughters but **none** of his sons. * **Phosphate Diabetes:** Unlike nutritional rickets, this condition does **not** respond to Vitamin D supplementation alone; it requires oral phosphate and calcitriol [1]. * **Most common inheritance:** Most enzyme deficiencies are AR, while most structural protein defects are AD. Most bleeding disorders (except von Willebrand Disease) are XLR.

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

Organic aciduria. **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).

Q: K-F ring is pathognomonic of which of the following conditions?

Decreased ceruloplasmin level. **Explanation:** The **Kayser-Fleischer (K-F) ring** is a classic ophthalmological sign characterized by a golden-brown or greenish-brown pigment deposition in the **Descemet’s membrane** of the peripheral cornea. It is caused by the deposition of excess **copper**. **Why Option D is Correct:** The K-F ring is most famously associated with **Wilson’s Disease** (Hepatolenticular degeneration). In Wilson’s disease, there is a deficiency of ATP7B (a copper-transporting ATPase), leading to impaired biliary excretion of copper and a failure to incorporate copper into ceruloplasmin. This results in **decreased serum ceruloplasmin levels** and the accumulation of free copper in various organs, including the liver, brain (basal ganglia), and the cornea (K-F ring). **Analysis of Incorrect Options:** * **A & B (Cholestatic Jaundice/PBC):** While K-F rings can rarely be seen in chronic cholestatic diseases (due to impaired biliary excretion of copper), they are not pathognomonic for these conditions. Wilson’s disease remains the primary association for exam purposes. * **C (Hemochromatosis):** This is a disorder of **iron** overload, not copper. Iron deposition in the eye typically presents as *siderosis bulbi*, not K-F rings. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** K-F rings start at the **Schwalbe’s line** and involve the **Descemet’s membrane**. * **Appearance:** They usually appear first in the superior pole, then inferior, and finally become circumferential. * **Detection:** While visible to the naked eye in advanced cases, a **Slit-lamp examination** is the gold standard for early detection. * **Reversibility:** The ring may disappear with effective chelation therapy (e.g., D-Penicillamine). * **Sunflower Cataract:** Another ocular feature of Wilson’s disease involving copper deposition in the anterior lens capsule.

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

Gaucher disease. **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.

Q: Mitochondrial DNA (mt-DNA) is known for all except:

Nemaline myopathy results due to mutations in mt-DNA. ### Explanation **Why Option D is the Correct Answer:** Nemaline myopathy is **not** a mitochondrial disorder. It is a heterogeneous group of congenital myopathies characterized by the presence of "nemaline rods" (thread-like structures) in muscle fibers. It is caused by mutations in genes encoding proteins of the **skeletal muscle thin filaments** (e.g., *NEB, ACTA1, TPM3*). These mutations follow **Autosomal Dominant or Recessive** inheritance patterns, rather than mitochondrial inheritance. **Analysis of Other Options:** * **A. Maternal inheritance:** This is a hallmark of mt-DNA. During fertilization, the sperm's mitochondria are typically degraded; thus, all mitochondria in the zygote are derived from the oocyte. * **B. Heteroplasmy:** This refers to the presence of a mixture of more than one type of organellar genome (normal and mutated mt-DNA) within a single cell. The severity of mitochondrial diseases often depends on the ratio of mutant to wild-type DNA. * **C. Leber Hereditary Optic Neuropathy (LHON):** This is the classic prototype of mitochondrial diseases. It is characterized by bilateral, painless subacute visual loss due to mutations in genes encoding subunits of Complex I (NADH dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Bottleneck Effect:** Explains the variability in the percentage of mutant mt-DNA passed from mother to offspring. * **Threshold Effect:** Clinical symptoms of mitochondrial diseases appear only when the proportion of mutated mt-DNA exceeds a specific limit. * **Common Mitochondrial Disorders:** MERRF (Myoclonic Epilepsy with Ragged Red Fibers), MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes), and Kearns-Sayre Syndrome. * **Histology:** "Ragged Red Fibers" on Gomori trichrome stain are characteristic of mitochondrial myopathies due to compensatory proliferation of mitochondria.

Q: Which of the following is an autosomal dominant metabolic disorder?

Hereditary hypercholesterolemia. **Explanation:** The correct answer is **Hereditary Hypercholesterolemia** (specifically Familial Hypercholesterolemia). **1. Why it is correct:** Most metabolic disorders (inborn errors of metabolism) are inherited in an **autosomal recessive** pattern because a 50% reduction in enzyme activity in heterozygotes is usually sufficient for normal function. However, **Familial Hypercholesterolemia (FH)** is a notable exception. It is an **autosomal dominant** disorder caused by mutations in the **LDL receptor gene**. In this case, a 50% reduction in receptors (heterozygous state) is insufficient to clear LDL from the plasma, leading to premature atherosclerosis and xanthomas. **2. Why the other options are incorrect:** * **Tay-Sachs Disease:** An autosomal recessive lysosomal storage disorder caused by a deficiency of **Hexosaminidase A**, leading to GM2 ganglioside accumulation. * **Gaucher’s Disease:** The most common lysosomal storage disorder, inherited in an **autosomal recessive** manner, caused by a deficiency of **glucocerebrosidase**. * **Tyrosinemia:** A group of **autosomal recessive** metabolic errors in the phenylalanine-tyrosine catabolic pathway (e.g., deficiency of fumarylacetoacetate hydrolase in Type I). **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Autosomal Dominant Metabolic Disorders:** Remember **"H-A-P-P-Y"**: **H**ereditary Spherocytosis/Hypercholesterolemia, **A**cute Intermittent Porphyria, **P**olycystic Kidney Disease (ADPKD), **P**seudohypoparathyroidism, **Y** (and others like Huntington’s). * **Key Clinical Feature of FH:** Tendon xanthomas (especially the Achilles tendon) and xanthelasma. * **Rule of Thumb:** Most enzyme deficiencies are Recessive; most structural protein or receptor defects are Dominant. **Acute Intermittent Porphyria** is another high-yield metabolic exception that is Autosomal Dominant.

Question 1

All of the following are mitochondrial disorders EXCEPT?

Accepted Answer

Incontinentia pigmenti

Answer

MELAS

Answer

Kearns Sayre syndrome

Answer

NARP syndrome

Question 2

Which one of the following disorders is autosomal recessive?

Accepted Answer

Homocystinuria

Answer

G6PD deficiency

Answer

Myotonic dystrophy

Answer

Otospongiosis

Question 3

Which of the following is an X-linked dominant condition?

Accepted Answer

Phosphate diabetes

Answer

Hemophilia

Answer

Gaucher's disease

Answer

Cystic fibrosis

Question 4

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

Accepted Answer

Organic aciduria

Answer

Mitochondrial encephalopathy with lactic aciduria

Answer

Phenylketonuria

Answer

Urea cycle enzyme deficiency

Question 5

K-F ring is pathognomonic of which of the following conditions?

Accepted Answer

Decreased ceruloplasmin level

Answer

Cholestatic jaundice

Answer

Primary biliary cholangitis (PBC)

Answer

Hemochromatosis

Question 6

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

Accepted Answer

Gaucher disease

Answer

Tay-Sachs disease

Answer

Fabry disease

Answer

Niemann-Pick disease

Question 7

Mitochondrial DNA (mt-DNA) is known for all except:

Accepted Answer

Nemaline myopathy results due to mutations in mt-DNA

Answer

Maternal inheritance

Answer

Heteroplasmy

Answer

Leber hereditary optic neuropathy is the prototype

Question 8

Which of the following is an autosomal dominant metabolic disorder?

Accepted Answer

Hereditary hypercholesterolemia

Answer

Tay-Sachs disease

Answer

Gaucher's disease

Answer

Tyrosinemia

Question 9

Metabolic abnormalities in which jaundice is seen are all of the following except?

Accepted Answer

Von Gierke disease

Answer

Galactosemia

Answer

Tyrosinemia

Answer

Hereditary fructose intolerance

Question 10

Abnormality in Vitamin D metabolism leading to rickets is associated with which of the following?

Accepted Answer

Loss of HCO3-

Answer

Loss of Ca++

Answer

Loss of K+

Answer

All of the above

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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