Mitochondrial Diseases — MCQs

10 questions

The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?

Which of the following processes does not occur in mitochondria?

An affected male does not have affected children but an affected female always has affected children. Type of inheritance?

A muscle biopsy shows 'moth-eaten' fibers. Which histochemical finding would confirm mitochondrial myopathy?

Mark the false statement regarding mitochondrial DNA:

A 48-year-old right-handed man was admitted with a 3-day history of brief generalized tonic-clonic seizures, muscle weakness, a 3-month history of weight loss with increasing difficulty in eating, and a 1-week history of word finding difficulties and speech apraxia with reduced fine motor skills. He had a diagnosis of type 2 diabetes mellitus, epilepsy, and a history of a left temporal lobe infarct 3 years ago. MRI of the brain showed bilateral cortical-based signal abnormality with associated edema, with corresponding diffusion hyperintensity. MR spectroscopy showed a lactate doublet peak. HPE of muscle biopsy was shown below. What is the most probable diagnosis?

Which of the following conditions is characterized by the presence of telangiectasia?

Which pattern of inheritance of disease is associated with consanguinity?

Which one of the following is an autosomal recessive disorder?

Q10

Which of the following is an Autosomal Dominant disease?

Mitochondrial Diseases Indian Medical PG Practice Questions and MCQs

Question 1: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?

A. Cyanide (Correct Answer)
B. Carbon dioxide
C. Oligomycin
D. Ouabain

Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.

Question 2: Which of the following processes does not occur in mitochondria?

A. Fatty acid oxidation
B. Electron transport chain
C. Glycogenolysis (Correct Answer)
D. Citric acid cycle (Kreb's cycle)

Explanation: ***Glycogenolysis*** - **Glycogenolysis** is the breakdown of **glycogen** into glucose, which primarily occurs in the **cytosol** of cells, mainly in the liver and muscles. - This process is crucial for maintaining blood glucose levels and providing energy during periods of fasting or increased demand, and it does not take place within the mitochondria. *Fatty acid oxidation* - **Fatty acid oxidation**, also known as beta-oxidation, is a mitochondrial process that breaks down fatty acids into **acetyl-CoA** for energy production. - This occurs extensively within the mitochondrial matrix, producing ATP. *Electron transport chain* - The **electron transport chain** is located in the **inner mitochondrial membrane** and is the final stage of aerobic respiration, producing the majority of ATP. - It involves a series of protein complexes that transfer electrons to oxygen, creating a proton gradient for ATP synthesis. *Citric acid cycle (Kreb's cycle)* - The **citric acid cycle**, or **Krebs cycle**, is a central metabolic pathway that occurs in the **mitochondrial matrix**. - It oxidizes acetyl-CoA, derived from carbohydrates, fats, and proteins, to produce ATP, NADH, and FADH2.

Question 3: An affected male does not have affected children but an affected female always has affected children. Type of inheritance?

A. Autosomal recessive
B. Mitochondrial (Correct Answer)
C. X linked recessive
D. X linked dominant

Explanation: ***Correct Option: Mitochondrial*** - This pattern describes **mitochondrial inheritance**, where all children of an **affected mother** inherit the condition because mitochondria are exclusively inherited from the ovum (maternal inheritance). - An **affected father** cannot pass on the condition to his children, as sperm contribute only nuclear DNA and essentially no mitochondria. - This is the **only inheritance pattern** where an affected male has no affected children while an affected female has all children affected. *Incorrect Option: Autosomal recessive* - This pattern would typically show affected individuals having unaffected parents (who are carriers) and both males and females being affected in equal proportions. - It does not explain the complete absence of transmission from an affected father or universal transmission from an affected mother. - An affected individual could have unaffected children if their partner is not a carrier. *Incorrect Option: X linked recessive* - In **X-linked recessive inheritance**, affected males cannot pass the trait to their sons, but all their daughters would be carriers (not affected). - An affected mother would pass the trait to all her sons (affected) and make all her daughters carriers (not affected), which does not match the described pattern of all children being affected. *Incorrect Option: X linked dominant* - In **X-linked dominant inheritance**, an affected father passes the trait to all his daughters but none of his sons (contradicts "no affected children"). - An affected mother has a 50% chance of passing the trait to **each child**, which is inconsistent with all children of an affected female being affected.

Question 4: A muscle biopsy shows 'moth-eaten' fibers. Which histochemical finding would confirm mitochondrial myopathy?

A. Ragged red fibers on MGT (Correct Answer)
B. Type 1 fiber predominance
C. Rimmed vacuoles
D. Nemaline rods

Explanation: ***Ragged red fibers on MGT*** - The presence of **ragged red fibers** on a **modified Gomori trichrome (MGT) stain** is the histological hallmark of **mitochondrial myopathies**. - These fibers represent abnormal accumulation of **dysfunctional mitochondria** beneath the sarcolemma. *Type 1 fiber predominance* - While some myopathies may show type 1 fiber predominance, it is a **non-specific finding** and does not confirm mitochondrial myopathy. - It can be seen in various conditions, including **neurogenic atrophy** or some **congenital myopathies**. *Rimmed vacuoles* - **Rimmed vacuoles** are characteristic of **inclusion body myositis** (IBM) and some **distal myopathies**. - They are not a specific finding for mitochondrial myopathy. *Nemaline rods* - **Nemaline rods** are pathognomonic for **nemaline rod myopathy**, a distinct form of congenital myopathy. - They are composed of **actin filament aggregates** and are unrelated to mitochondrial dysfunction.

Question 5: Mark the false statement regarding mitochondrial DNA:

A. AGA and AGG are stop codons in mitochondrial DNA
B. Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA
C. Does not show heteroplasmy (Correct Answer)
D. 1% of cellular DNA, 13 proteins of respiratory chain

Explanation: ***Does not show heteroplasmy*** - This statement is false because **mitochondrial DNA (mtDNA)** commonly exhibits **heteroplasmy**, meaning the presence of more than one type of mitochondrial genome within a cell or individual. - **Heteroplasmy** arises due to the presence of both normal and mutated mtDNA, which can be passed down from the mother. *AGA and AGG are stop codons in mitochondrial DNA* - This statement is true; in the **universal genetic code**, AGA and AGG code for **arginine**, but in **human mitochondrial DNA**, they serve as **stop codons**. - This is an example of the **differences** in genetic code interpretation between the nuclear genome and the mitochondrial genome. *Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA* - This statement is true; **Kearns-Sayre Syndrome** is a well-known mitochondrial disorder caused by a **large single deletion** in the mitochondrial DNA. - This deletion often leads to chronic progressive **external ophthalmoplegia**, **retinal pigmentary degeneration**, and **cardiac conduction defects**. *1% of cellular DNA, 13 proteins of respiratory chain* - This statement is true; **mitochondrial DNA constitutes** approximately **1% of the total cellular DNA** by mass. - It codes for **13 essential proteins** that are part of the **electron transport chain** (respiratory chain) complexes in the mitochondrion, along with ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs).

Question 6: A 48-year-old right-handed man was admitted with a 3-day history of brief generalized tonic-clonic seizures, muscle weakness, a 3-month history of weight loss with increasing difficulty in eating, and a 1-week history of word finding difficulties and speech apraxia with reduced fine motor skills. He had a diagnosis of type 2 diabetes mellitus, epilepsy, and a history of a left temporal lobe infarct 3 years ago. MRI of the brain showed bilateral cortical-based signal abnormality with associated edema, with corresponding diffusion hyperintensity. MR spectroscopy showed a lactate doublet peak. HPE of muscle biopsy was shown below. What is the most probable diagnosis?

A. Myotonic dystrophy
B. Duchenne muscular dystrophy
C. MELAS disease (Correct Answer)
D. Becker's muscular dystrophy

Explanation: ***MELAS disease*** - The clinical picture of **seizures**, **muscle weakness**, **stroke-like episodes** (indicated by cortical signal abnormalities and prior infarct), **weight loss**, and **speech difficulties** are characteristic features of MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes). - The **lactate doublet peak on MR spectroscopy** confirms lactic acidosis, a hallmark of mitochondrial disorders, and the muscle biopsy showing **ragged red fibers** (as depicted in the image) is pathognomonic for MELAS. *Myotonic dystrophy* - Characterized by **myotonia** (delayed muscle relaxation), **muscle wasting**, and involvement of other systems such as cataracts and cardiac conduction defects. - While it causes muscle weakness, it does not typically present with stroke-like episodes, lactic acidosis, or the specific MRI findings seen in this case. *Duchenne muscular dystrophy* - This is an **X-linked recessive disorder** primarily affecting young boys, causing progressive muscle degeneration and weakness, typically starting in early childhood. - It would not present with adult-onset stroke-like episodes, lactic acidosis, or the neurological features described in this patient. *Becker's muscular dystrophy* - It is a milder variant of Duchenne muscular dystrophy with a later onset and slower progression, but it also primarily affects skeletal and cardiac muscles. - Like Duchenne, it does not typically cause stroke-like episodes, lactic acidosis, or the specific MRI and spectroscopy findings observed in this patient.

Question 7: Which of the following conditions is characterized by the presence of telangiectasia?

A. Hereditary hemorrhagic telangiectasia (Correct Answer)
B. Telangiectasia due to systemic sclerosis
C. Telangiectasia due to rosacea
D. Acquired telangiectasia due to sun exposure

Explanation: ***Hereditary hemorrhagic telangiectasia*** - This condition, also known as **Osler-Weber-Rendu disease**, is a **genetic disorder** resulting in abnormal blood vessel formation, leading to **telangiectasias** and arteriovenous malformations that can cause significant bleeding. - Key diagnostic criteria include spontaneous recurrent nosebleeds, multiple mucocutaneous telangiectasias, visceral organ involvement (e.g., pulmonary, hepatic, cerebral AVMs), and a family history. *Telangiectasia due to systemic sclerosis* - While telangiectasias can be a feature of **systemic sclerosis** (especially the limited cutaneous form, **CREST syndrome**), they are typically localized and are not the primary defining characteristic of the disease. - Systemic sclerosis is primarily characterized by **fibrosis of skin and internal organs**, and the telangiectasias are a secondary manifestation, not the fundamental underpinning of the condition as in HHT. *Telangiectasia due to rosacea* - **Rosacea** is a chronic inflammatory skin condition characterized by facial erythema, papules, pustules, and **telangiectasias**, particularly on the cheeks and nose. - However, rosacea-associated telangiectasias are localized to the face and are part of a broader inflammatory dermatological process, distinct from the systemic vascular abnormalities seen in HHT. *Acquired telangiectasia due to sun exposure* - **Sun exposure** can indeed cause **telangiectasias**, particularly on sun-damaged skin, due to chronic photodamage to dermal blood vessels. - These are generally localized, non-syndromic, and a result of environmental factors rather than a systemic or inherited disorder as seen in hereditary hemorrhagic telangiectasia.

Question 8: Which pattern of inheritance of disease is associated with consanguinity?

A. Autosomal dominant
B. X linked recessive
C. Autosomal recessive (Correct Answer)
D. X linked dominant

Explanation: ***Autosomal recessive*** - Consanguineous relationships increase the likelihood of offspring inheriting **two copies of a rare deleterious recessive allele**, one from each parent [1]. - This occurs because relatives share a greater proportion of their genes, making it more probable that both parents are **heterozygous carriers** for the same recessive disorder. *Autosomal dominant* - In autosomal dominant disorders, only **one copy of the altered gene** is needed to cause the disease, so expression is not typically influenced by consanguinity. - These conditions often manifest in every generation and are not more prevalent with increased shared genetic material. *X-linked recessive* - X-linked recessive disorders primarily affect males, as they only have one X chromosome, and are transmitted by carrier females. - While consanguinity can theoretically increase the carrier rate within a population, its direct impact on the inheritance pattern of an X-linked trait in a single family is less pronounced compared to autosomal recessive conditions. *X-linked dominant* - X-linked dominant disorders are expressed in heterozygous females and hemizygous males, with affected males often having more severe disease. - The inheritance pattern is not significantly influenced by consanguinity, as only one copy of the affected gene on the X chromosome is sufficient for disease manifestation.

Question 9: Which one of the following is an autosomal recessive disorder?

A. Cystic fibrosis (Correct Answer)
B. Huntington's disease
C. Marfan syndrome
D. Neurofibromatosis type 1

Explanation: ***Cystic fibrosis*** - **Cystic fibrosis** is caused by mutations in the **CFTR gene**, leading to defective **chloride channel** function. - It is an **autosomal recessive disorder**, meaning an individual must inherit two copies of the mutated gene (one from each parent) to be affected. *Huntington's disease* - **Huntington's disease** is an **autosomal dominant disorder** caused by a mutation in the HTT gene. - Only one copy of the mutated gene is sufficient to cause the disease, resulting in progressive neurodegeneration. *Marfan syndrome* - **Marfan syndrome** is an **autosomal dominant disorder** affecting connective tissue, caused by mutations in the FBN1 gene. - It results in skeletal, ocular, and cardiovascular abnormalities due to defective **fibrillin-1**. *Neurofibromatosis type 1* - **Neurofibromatosis type 1** is an **autosomal dominant disorder** caused by mutations in the NF1 gene. - It is characterized by multiple neurofibromas, café-au-lait macules, and Lisch nodules.

Question 10: Which of the following is an Autosomal Dominant disease?

A. Wilson's disease
B. Von Willebrand disease (Correct Answer)
C. Cystic fibrosis
D. Friedreich's ataxia

Explanation: ***Von Willebrand disease*** - This is an **autosomal dominant** inherited disorder caused by a deficiency or dysfunction of **von Willebrand factor**, crucial for normal hemostasis [1], [2]. - It often presents with **mucocutaneous bleeding** like nosebleeds, easy bruising, and heavy menstrual bleeding [2]. *Wilson's disease* - This is an **autosomal recessive** disorder characterized by defective copper metabolism and accumulation in various organs, most notably the liver and brain. - Clinical features include **hepatic dysfunction**, neurologic symptoms like **tremors**, and **Kayser-Fleischer rings** in the eyes. *Cystic fibrosis* - This is an **autosomal recessive** disorder caused by mutations in the **CFTR gene**, leading to abnormal chloride transport. - It primarily affects the lungs and digestive system, causing **thick mucus production** and recurrent infections. *Friedreich's ataxia* - This is an **autosomal recessive** neurodegenerative disorder typically caused by a **GAA trinucleotide repeat expansion** in the FXN gene. - Key symptoms include progressive **ataxia**, dysarthria, and often heart problems.

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