Molecular Physiology — MCQs

Molecular Physiology Indian Medical PG Practice Questions and MCQs

Question 11: Which receptor binds to Brain-Derived Neurotrophic Factor (BDNF)?

A. TrK-A
B. TrK-B (Correct Answer)
C. TrK-C
D. None of the above

Explanation: ### Explanation **Neurotrophins** are a family of proteins essential for the survival, development, and function of neurons. They exert their effects by binding to specific high-affinity transmembrane tyrosine kinase receptors known as **TrK (Tropomyosin receptor kinase)** receptors. **Why TrK-B is the Correct Answer:** Each neurotrophin has a specific affinity for a particular TrK receptor. **Brain-Derived Neurotrophic Factor (BDNF)** and **Neurotrophin-4/5 (NT-4/5)** specifically bind to **TrK-B**. Upon binding, BDNF triggers dimerization and autophosphorylation of the receptor, activating downstream signaling pathways (like MAPK and PI3K) that promote neuronal plasticity, long-term potentiation (LTP), and cell survival. **Analysis of Incorrect Options:** * **Option A (TrK-A):** This is the primary receptor for **Nerve Growth Factor (NGF)**. It is crucial for the survival of sympathetic and sensory neurons. * **Option C (TrK-C):** This receptor specifically binds to **Neurotrophin-3 (NT-3)**. NT-3 is unique because it can also bind to TrK-A and TrK-B with lower affinity, but TrK-C is its primary target. **High-Yield NEET-PG Pearls:** 1. **p75NTR Receptor:** All neurotrophins (NGF, BDNF, NT-3, NT-4/5) bind with low affinity to the **p75 neurotrophin receptor**, which often mediates apoptosis (cell death) rather than survival. 2. **BDNF & Memory:** BDNF is highly expressed in the **hippocampus** and is a key molecular mediator of synaptic plasticity and memory formation. 3. **Clinical Correlation:** Reduced levels of BDNF are implicated in the pathogenesis of **Depression** and **Alzheimer’s disease**. Antidepressants often work by increasing BDNF expression. 4. **Mnemonic:** * **A**-NGF (A comes first, like NGF discovery) * **B**-BDNF (B for B) * **C**-NT-3 (C is the 3rd letter)

Question 12: Which of the following is not a channelopathy?

A. Cystic fibrosis
B. Liddle's syndrome
C. Tay-sach's disease (Correct Answer)
D. Hypokalemic periodic paralysis

Explanation: **Explanation:** **Channelopathies** are a group of disorders caused by mutations in genes encoding ion channel proteins or their regulatory subunits, leading to dysfunctional ion transport across cell membranes. **Why Tay-Sachs Disease is the Correct Answer:** Tay-Sachs disease is **not** a channelopathy; it is a **Lysosomal Storage Disorder**. It is caused by a deficiency of the enzyme **Hexosaminidase A**, leading to the toxic accumulation of GM2 gangliosides in neurons. Clinically, it presents with neurodegeneration, developmental delay, and a characteristic **cherry-red spot** on the macula. **Analysis of Incorrect Options:** * **Cystic Fibrosis:** This is a classic channelopathy caused by mutations in the **CFTR gene**, which encodes a cAMP-regulated **Chloride channel**. Dysfunction leads to thick, viscid secretions in the lungs and pancreas. * **Liddle’s Syndrome:** A genetic form of hypertension caused by a "gain-of-function" mutation in the **ENaC (Epithelial Sodium Channel)** in the collecting ducts of the kidney, leading to excessive sodium reabsorption. * **Hypokalemic Periodic Paralysis:** A skeletal muscle channelopathy most commonly involving the **voltage-gated Calcium channel (Cav1.1)** or Sodium channel (Nav1.4), resulting in episodes of muscle weakness triggered by low serum potassium. **High-Yield Clinical Pearls for NEET-PG:** * **Common Channelopathies to Remember:** Lambert-Eaton Syndrome (Ca²⁺ channel), Myasthenia Gravis (ACh-gated Na⁺ channel), and Long QT Syndrome (K⁺/Na⁺ channels). * **Bartter’s and Gitelman’s Syndromes** are also important renal channelopathies involving salt transporters. * **Tay-Sachs Mnemonic:** "A **Gang** of **Six** (**Hex**osaminidase) **Small** (**Tay**-Sachs) **Cherries** (Cherry-red spot)."

Question 13: Which of the following conditions CAN BE transmitted as a recessive, sex-linked trait? I. Retinitis pigmentosa II. Colour blindness III. Cystic fibrosis IV. Duchenne muscular dystrophy Select the correct answer using the code given below :

A. II, III and IV
B. I, II and IV (Correct Answer)
C. I, II and III
D. I, III and IV

Explanation: ***I, II and IV*** - All three conditions listed—**Retinitis Pigmentosa**, **Colour Blindness**, and **Duchenne Muscular Dystrophy**—can be inherited as recessive, **sex-linked traits**. - **Sex-linked inheritance** primarily refers to genes located on the **X chromosome**; males are more frequently affected because they have only one X chromosome. *II, III and IV* - This option incorrectly includes **Cystic Fibrosis**, which is an **autosomal recessive disorder**, not a sex-linked trait. - While **Colour Blindness** and **Duchenne Muscular Dystrophy** are sex-linked, their grouping with an autosomal condition makes this option incorrect. *I, II and III* - This option also incorrectly includes **Cystic Fibrosis** as a sex-linked trait. - **Retinitis Pigmentosa** and **Colour Blindness** can be sex-linked, but the inclusion of **Cystic Fibrosis** renders the entire option incorrect. *I, III and IV* - This option incorrectly includes **Cystic Fibrosis**; it is an **autosomal recessive condition**, not sex-linked. - Although **Retinitis Pigmentosa** and **Duchenne Muscular Dystrophy** are correctly identified as conditions that can be sex-linked, the incorrect inclusion of **Cystic Fibrosis** makes this option wrong.

Question 14: Which one of the following correctly denotes the inheritance pattern of cystic fibrosis?

A. X-linked Dominant
B. Autosomal Recessive (Correct Answer)
C. Autosomal Dominant
D. X-linked Recessive

Explanation: ***Autosomal Recessive*** - Cystic fibrosis is an **autosomal recessive** disorder, meaning an individual must inherit **two copies** of the defective gene (one from each parent) to develop the condition. - Parents who are **carriers** (having one normal and one defective gene) typically do not show symptoms but can pass the gene to their children. *X-linked Dominant* - In **X-linked dominant** inheritance, a single copy of a mutated gene on the X chromosome is sufficient to cause the disorder. - This pattern would show common inheritance in females and often more severe phenotypes in males, which is not characteristic of cystic fibrosis. *Autosomal Dominant* - **Autosomal dominant** disorders require only **one copy** of a mutated gene on a non-sex chromosome for the disease to manifest. - If cystic fibrosis were autosomal dominant, affected individuals would typically have an affected parent, and the disease would be much more prevalent than observed. *X-linked Recessive* - **X-linked recessive** disorders primarily affect males, as they have only one X chromosome. Females are typically carriers and less severely affected. - Cystic fibrosis affects males and females nearly equally, which rules out an X-linked recessive inheritance pattern.

Question 15: Otosclerosis is inherited in which manner?

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

Explanation: ***Autosomal dominant*** - **Otospongiosis**, also known as **otosclerosis**, is most frequently inherited in an autosomal dominant pattern with **incomplete penetrance**. - This means that a single copy of the altered gene is sufficient to cause the condition, but not everyone who inherits the gene will develop symptoms. *Autosomal recessive* - **Autosomal recessive** inheritance requires two copies of the altered gene (one from each parent) for the condition to manifest. - This pattern is less common for otospongiosis and not considered the primary mode of inheritance. *X-linked dominant* - **X-linked dominant** inheritance affects both males and females, but typically males are more severely affected than females. - While otospongiosis shows some sex differences in presentation, its inheritance pattern is not primarily linked to the X chromosome in a dominant fashion. *X-linked recessive* - **X-linked recessive** inheritance primarily affects males since they only have one X chromosome, and females are typically carriers. - This pattern is not characteristic of otospongiosis, as evidence shows a significant transmission to both sexes from affected individuals.

Question 16: The gene that regulates normal morphogenesis during development is?

A. FMR-1 gene
B. P-16
C. Homeobox gene (Correct Answer)
D. PTEN

Explanation: ***Homeobox gene*** - **Homeobox genes** (Hox genes) are a large family of genes that play a critical role in regulating the **patterning of the body axis** and the **development of organs** and appendages during embryonic development. - They encode **transcription factors** that control the expression of other genes involved in morphogenesis. *FMR-1 gene* - The **FMR-1 gene** (fragile X mental retardation 1 gene) is associated with **Fragile X syndrome**, a genetic condition causing intellectual disability and developmental problems. - Its primary role is in **brain development** and synaptic function, not general normal morphogenesis. *P-16* - **P16 (CDKN2A)** is a **tumor suppressor gene** involved in regulating the cell cycle by inhibiting cyclin-dependent kinases. - Its main function is in preventing uncontrolled cell growth and proliferation, not directly in orchestrating embryonic morphogenesis. *PTEN* - **PTEN** is another prominent **tumor suppressor gene** that plays a crucial role in cell growth, survival, and proliferation. - Mutations in PTEN are linked to various cancers and developmental disorders such as **Cowden syndrome**, but its primary function is not in regulating the broad aspects of normal embryonic morphogenesis.

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