Hemoglobin and Iron Metabolism — MCQs

A 20-year-old woman complains of intermittent, colicky abdominal pain, fine tremors of her hands, excess sweating, and a general feeling of restlessness. Laboratory studies reveal an inherited defect in the biosynthesis of heme. This patient's genetic disease is most likely caused by deficiency of which of the following liver enzymes?

Q79Easy

2,3-BPG binds to sites on hemoglobin and the affinity of hemoglobin for oxygen:

Q80Easy

Immunoglobulins are:

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 71: The 'Hbs' mutation in hemoglobin involves a defect at which amino acid position?

A. 4
B. 6 (Correct Answer)
C. 11
D. 12

Explanation: **Explanation:** **1. Why Option B is Correct:** Sickle Cell Hemoglobin (HbS) is caused by a **point mutation** (specifically a transversion) in the **$\beta$-globin chain**. The defect occurs at the **6th position** of the $\beta$-chain, where the amino acid **Glutamic acid** (polar/negatively charged) is replaced by **Valine** (non-polar/hydrophobic). * **Mechanism:** Under deoxygenated conditions, the hydrophobic valine at position 6 creates a "sticky patch" that interacts with complementary sites on other hemoglobin molecules. This leads to polymerization, forming long fibers that distort the RBC into a sickle shape. **2. Why Other Options are Incorrect:** * **Option A (4):** No major hemoglobinopathy is associated with the 4th position of the $\beta$-chain. * **Option C (11):** While mutations can occur anywhere, position 11 is not the site for HbS, HbC, or HbE. * **Option D (12):** Position 12 is not associated with the common sickle cell mutation. **3. High-Yield Clinical Pearls for NEET-PG:** * **HbC Disease:** Also occurs at the **6th position** of the $\beta$-chain, but Glutamic acid is replaced by **Lysine** (Mnemonic: **L**ysine is **C**harged). * **HbE Disease:** Occurs at the **26th position** ($\beta$-chain), where Glutamic acid is replaced by Lysine. * **Electrophoresis:** On alkaline electrophoresis, HbS moves slower than HbA because it loses a negative charge (Glutamic acid). HbC moves even slower than HbS. * **Inheritance:** It is an autosomal recessive disorder. The mutation is a **GAG to GTG** change in the DNA sequence.

Question 72: Ferritin, an inactive form of iron, is stored in which organ?

A. Gut
B. Spleen
C. Liver (Correct Answer)
D. All of the above

Explanation: ### Explanation **Core Concept:** Iron is a vital but toxic element; therefore, the body stores it in a non-toxic, water-soluble protein complex called **Ferritin**. While ferritin is present in almost all cells, the **Liver** is the primary anatomical site for iron storage. Within the liver, iron is stored in hepatocytes and Kupffer cells. When iron levels exceed the storage capacity of ferritin, it is converted into **hemosiderin**, an insoluble form. **Analysis of Options:** * **Option C (Liver):** This is the correct answer. The liver acts as the main reservoir for iron in the body. Serum ferritin levels are generally directly proportional to the total body iron stores held within the liver. * **Option A (Gut):** The enterocytes of the small intestine (duodenum) are responsible for iron **absorption**, not long-term storage. Iron is either transported into the blood via ferroportin or lost when the mucosal cells slough off. * **Option B (Spleen):** While the spleen contains macrophages that recycle iron from old red blood cells, it is a secondary site of storage compared to the liver. * **Option D (All of the above):** Incorrect, as the liver is the definitive primary storage organ. **NEET-PG High-Yield Pearls:** 1. **Apoferritin vs. Ferritin:** Apoferritin is the protein shell; once it binds with ferric iron ($Fe^{3+}$), it is called Ferritin. 2. **Best Screening Test:** Serum ferritin is the most sensitive and specific first-line index for diagnosing **Iron Deficiency Anemia** (it decreases before hemoglobin drops). 3. **Acute Phase Reactant:** Ferritin levels rise during inflammation, infection, or malignancy, which can mask an underlying iron deficiency. 4. **Prussian Blue Stain:** This is the specific stain used to visualize iron (ferritin/hemosiderin) in tissue biopsies (e.g., bone marrow or liver).

Question 73: In the first step of heme synthesis, which amino acid is required?

A. Histidine
B. Folate
C. Glycine (Correct Answer)
D. Iron

Explanation: ### Explanation **Correct Option: C (Glycine)** The biosynthesis of heme begins in the **mitochondria**. The first and rate-limiting step involves the condensation of **Succinyl-CoA** (from the TCA cycle) and the amino acid **Glycine**. This reaction is catalyzed by the enzyme **ALA Synthase (ALAS)** and requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. The product formed is $\delta$-aminolevulinic acid (ALA). **Analysis of Incorrect Options:** * **A. Histidine:** While Histidine is crucial for hemoglobin function (the proximal and distal histidines bind iron and oxygen), it is not a substrate in the synthesis of the heme ring itself. * **B. Folate:** Folate is essential for DNA synthesis and erythropoiesis. A deficiency leads to megaloblastic anemia, but it does not serve as a building block for the heme molecule. * **D. Iron:** Iron is the central metal atom of the heme group, but it is incorporated in the **final step** of the pathway. The enzyme **Ferrochelatase** inserts ferrous iron ($Fe^{2+}$) into Protoporphyrin IX in the mitochondria. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** ALA Synthase 1 (liver) is inhibited by heme (feedback inhibition), while ALA Synthase 2 (erythroid cells) is regulated by iron availability. * **Cofactor Dependency:** Vitamin B6 deficiency can lead to **Sideroblastic Anemia** because the first step of heme synthesis is impaired, causing iron to sequester in mitochondria (forming ringed sideroblasts). * **Lead Poisoning:** Lead inhibits two enzymes in this pathway: **ALA Dehydratase** (2nd step) and **Ferrochelatase** (last step). * **Location:** Heme synthesis occurs partly in the mitochondria (1st and last three steps) and partly in the cytosol. Memory aid: *"The first and the last are in the mitochondria."*

Question 74: What are the hemoglobin types found in a normal adult?

A. Hemoglobin S, A, and F
B. Hemoglobin A, A2, and C
C. Hemoglobin A2 and F
D. Hemoglobin A, A2, and F (Correct Answer)

Explanation: ### Explanation In a healthy adult, hemoglobin (Hb) is not a single entity but a mixture of three distinct types, categorized by their globin chain composition. **1. Why Option D is Correct:** The normal adult hemoglobin profile consists of: * **HbA ($\alpha_2\beta_2$):** The major component, comprising approximately **95–97%** of total hemoglobin. * **HbA2 ($\alpha_2\delta_2$):** A minor component, comprising **2–3%**. It is clinically significant as it increases in $\beta$-thalassemia trait. * **HbF ($\alpha_2\gamma_2$):** Fetal hemoglobin, which persists in trace amounts (**<1%**) in adults. **2. Analysis of Incorrect Options:** * **Option A & B:** These include **HbS** and **HbC**. These are pathological variants (hemoglobinopathies) resulting from point mutations in the $\beta$-globin gene. HbS causes Sickle Cell Disease, and HbC causes HbC disease. They are not found in a "normal" adult. * **Option C:** This option is incomplete as it omits **HbA**, which is the most abundant hemoglobin type in adults. **3. NEET-PG High-Yield Clinical Pearls:** * **Switching:** The transition from HbF to HbA (the "$\gamma$ to $\beta$ switch") begins before birth but is mostly completed by **6 months of age**. * **HbA1c:** This is a non-enzymatic glycation of HbA used to monitor long-term glycemic control in diabetic patients (reflecting the previous 90–120 days). * **Diagnostic Tool:** **Hemoglobin Electrophoresis** or HPLC is used to quantify these percentages; an HbA2 level >3.5% is a classic diagnostic marker for **$\beta$-Thalassemia minor**. * **Globin Chains:** All normal human hemoglobins (HbA, A2, F) share the **$\alpha$-chain**; they differ only in their non-$\alpha$ chains ($\beta, \delta, \gamma$).

Question 75: Each milliliter of red blood cells contains how many mg of elemental iron?

A. 1 mg (Correct Answer)
B. 2 mg
C. 3 mg
D. 4 mg

Explanation: ### Explanation **Correct Answer: A. 1 mg** The concentration of iron in red blood cells (RBCs) is directly linked to hemoglobin content. Under normal physiological conditions, **1 mL of packed red blood cells contains approximately 1 mg of elemental iron.** This relationship is derived from the following calculation: 1. Hemoglobin contains approximately **0.34% iron** by weight. 2. 100 mL of packed RBCs contains roughly 30–34 grams of hemoglobin. 3. Therefore, 100 mL of RBCs contains ~100 mg of iron, which simplifies to **1 mg of iron per 1 mL of RBCs.** **Analysis of Incorrect Options:** * **B, C, and D (2 mg, 3 mg, 4 mg):** These values overestimate the iron-carrying capacity of hemoglobin. While a unit of whole blood (approx. 350–450 mL) contains about 200–250 mg of iron, the specific ratio for pure RBC volume remains 1:1. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Donation:** Since 1 mL of RBCs contains 1 mg of iron, donating one unit of whole blood (which contains ~200–250 mL of RBCs) results in a loss of approximately **200–250 mg of iron.** * **Iron Storage:** Total body iron is roughly **3–4 grams**, with the majority (~65-70%) found in hemoglobin. * **Daily Requirement:** The average adult loses about 1 mg of iron daily (via shedding of skin and GI cells), which is balanced by the absorption of 1 mg from the diet. * **Transfusion Overload:** In patients with chronic transfusion requirements (e.g., Thalassemia), every 1 mL of packed RBCs transfused adds 1 mg of iron to the body. Since the body lacks an active excretory pathway for iron, this leads to **hemosiderosis** after approximately 10–20 units.

Question 76: Menke's disease is due to a defect in the metabolism of which element?

A. Zinc
B. Copper (Correct Answer)
C. Selenium
D. Iron

Explanation: **Explanation:** **Menkes Disease (Kinky Hair Syndrome)** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a copper-transporting ATPase responsible for the absorption of copper from the gastrointestinal tract and its distribution to various tissues. In Menkes disease, copper is trapped within intestinal mucosal cells, leading to severe systemic **copper deficiency**. Copper is a vital cofactor for several enzymes. Its deficiency leads to the characteristic clinical features: * **Lysyl Oxidase deficiency:** Causes defective collagen cross-linking, leading to arterial tortuosity and skeletal abnormalities. * **Tyrosinase deficiency:** Results in hypopigmentation. * **Cytochrome c oxidase deficiency:** Leads to neurodegeneration and hypothermia. * **Clinical Hallmark:** "Steely" or "Kinky" brittle hair due to defective keratin disulfide bond formation. **Why other options are incorrect:** * **Zinc:** Deficiency causes *Acrodermatitis enteropathica*, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Selenium:** Deficiency is associated with *Keshan disease* (dilated cardiomyopathy). * **Iron:** Metabolism defects lead to *Iron Deficiency Anemia* or *Hemochromatosis* (iron overload), not Menkes disease. **High-Yield NEET-PG Pearls:** 1. **ATP7A vs. ATP7B:** Remember **A** for **A**bsorption (Menkes/ATP7A - deficiency) and **B** for **B**iliary excretion (Wilson’s/ATP7B - overload). 2. **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. 3. **Inheritance:** Menkes is X-linked Recessive (mostly affects males), whereas Wilson’s is Autosomal Recessive.

Question 77: Vitamin K is involved in the action of which of the following proteins, except?

A. Factor II
B. Factor X
C. Factor I (Correct Answer)
D. Protein C

Explanation: **Explanation:** Vitamin K acts as a vital cofactor for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme catalyzes the post-translational modification of specific glutamic acid residues into **$\gamma$-carboxyglutamic acid (Gla)**. This modification allows these proteins to bind calcium ions ($Ca^{2+}$), which is essential for their binding to phospholipid membranes during the coagulation cascade. **Why Factor I is the correct answer:** **Factor I (Fibrinogen)** is a soluble plasma glycoprotein that is converted to fibrin by thrombin. Its synthesis and function are **independent of Vitamin K**. It does not undergo $\gamma$-carboxylation; therefore, Vitamin K deficiency or warfarin therapy does not affect its levels. **Analysis of incorrect options:** * **Factor II (Prothrombin) and Factor X:** These are Vitamin K-dependent procoagulant clotting factors synthesized in the liver. Other factors in this group include **Factor VII and Factor IX**. * **Protein C:** Along with **Protein S and Protein Z**, Protein C is a Vitamin K-dependent **anticoagulant** protein. It inhibits Factors Va and VIIIa. **NEET-PG High-Yield Pearls:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the synthesis of Factors II, VII, IX, X, and Proteins C and S. * **Half-life:** **Factor VII** has the shortest half-life, while **Protein C** is the first anticoagulant to decrease, which can lead to "Warfarin-induced skin necrosis" if not bridged with heparin. * **Laboratory Marker:** Prothrombin Time (PT/INR) is used to monitor Vitamin K status and extrinsic pathway activity.

Question 78: A 20-year-old woman complains of intermittent, colicky abdominal pain, fine tremors of her hands, excess sweating, and a general feeling of restlessness. Laboratory studies reveal an inherited defect in the biosynthesis of heme. This patient's genetic disease is most likely caused by deficiency of which of the following liver enzymes?

A. Alanine aminotransferase
B. Alkaline phosphatase
C. Porphobilinogen deaminase (Correct Answer)
D. Uridine diphosphate glucuronyl transferase

Explanation: ### Explanation The clinical presentation of **intermittent colicky abdominal pain**, neurological symptoms (**tremors, restlessness**), and autonomic instability (**sweating**) in a young woman suggests **Acute Intermittent Porphyria (AIP)**. AIP is an autosomal dominant metabolic disorder caused by a deficiency of **Porphobilinogen (PBG) deaminase** (also known as Hydroxymethylbilane synthase). #### Why Porphobilinogen Deaminase is Correct: In the heme biosynthetic pathway, PBG deaminase converts porphobilinogen to hydroxymethylbilane. A deficiency leads to the accumulation of toxic upstream precursors: **delta-aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**. These precursors are neurotoxic, leading to the "classic triad" of AIP: 1. **Abdominal pain** (most common, often severe and out of proportion to physical findings). 2. **Neuropsychiatric symptoms** (tremors, anxiety, psychosis). 3. **Autonomic dysfunction** (tachycardia, sweating, hypertension). #### Why Other Options are Incorrect: * **A. Alanine aminotransferase (ALT):** A marker of hepatocellular injury (e.g., hepatitis), not an enzyme in the heme synthesis pathway. * **B. Alkaline phosphatase (ALP):** A marker of cholestasis or bone turnover; its deficiency is seen in hypophosphatasia, not porphyria. * **D. UDP-glucuronyl transferase:** Deficiency of this enzyme leads to **Crigler-Najjar** or **Gilbert syndrome**, characterized by unconjugated hyperbilirubinemia (jaundice), not colicky abdominal pain or neurotoxicity. #### NEET-PG High-Yield Pearls: * **AIP Mnemonic (The 5 P's):** **P**ainful abdomen, **P**ort-wine colored urine (on standing), **P**olyneuropathy, **P**sychological disturbances, **P**recipitated by drugs (e.g., Barbiturates, Sulfonamides, Alcohol). * **Key Enzyme:** PBG Deaminase (Cytosolic enzyme). * **Diagnosis:** Elevated urinary ALA and PBG levels. * **Treatment:** Hemin and Glucose (both inhibit **ALA synthase**, the rate-limiting enzyme, via feedback inhibition). * **Important Distinction:** Unlike Porphyria Cutanea Tarda (PCT), AIP has **no photosensitivity**.

Question 79: 2,3-BPG binds to sites on hemoglobin and the affinity of hemoglobin for oxygen:

A. 4, decreases
B. 1, decreases (Correct Answer)
C. 4, increases
D. 1, increases

Explanation: ### Explanation **1. Why Option B is Correct:** 2,3-Bisphosphoglycerate (2,3-BPG) is a critical allosteric effector that stabilizes the **T-state (Tense/Deoxygenated)** of hemoglobin. It binds to a **single (1)** central cavity located between the two beta-globin chains. This cavity contains positively charged amino acids (Lysine and Histidine) that interact with the negatively charged phosphate groups of 2,3-BPG. By stabilizing the T-state, 2,3-BPG **decreases** the affinity of hemoglobin for oxygen, causing the oxygen dissociation curve to shift to the **right**. This facilitates the unloading of oxygen to peripheral tissues. **2. Why Other Options are Incorrect:** * **Options A & C (4 sites):** Hemoglobin has four heme groups for oxygen binding, but it possesses only **one** central pocket for 2,3-BPG. Binding at four sites is structurally impossible for this regulator. * **Options C & D (Increases affinity):** Increasing oxygen affinity would shift the curve to the left, making it harder for tissues to receive oxygen. Only factors like decreased temperature or increased pH (Bohr effect) increase affinity; 2,3-BPG is a physiological "unloader." **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Fetal Hemoglobin (HbF):** HbF has a **lower affinity** for 2,3-BPG because its gamma chains have Serine instead of Histidine at position 143. This results in a **higher oxygen affinity** for HbF, allowing the fetus to "pull" oxygen from maternal HbA. * **Adaptation to High Altitude:** Chronic hypoxia triggers an **increase** in 2,3-BPG levels to enhance oxygen delivery to tissues. * **Stored Blood:** 2,3-BPG levels drop in stored blood. Transfusing large amounts of "old" blood can cause a left shift, temporarily impairing oxygen delivery until the recipient's body regenerates 2,3-BPG.

Question 80: Immunoglobulins are:

A. Proteins
B. Glycoproteins (Correct Answer)
C. Proteoglycans
D. Glycosides

Explanation: **Explanation:** **Why Glycoproteins is the correct answer:** Immunoglobulins (antibodies) are specialized proteins produced by B-lymphocytes (plasma cells). Structurally, they consist of polypeptide chains (heavy and light chains) to which **carbohydrate moieties** are covalently attached. Specifically, these carbohydrates are linked to the constant regions of the heavy chains. Because the protein component significantly outweighs the carbohydrate component (usually 3–15% carbohydrate), they are classified as **Glycoproteins**. These sugar chains are crucial for maintaining the structural stability of the antibody and mediating effector functions like complement activation. **Analysis of Incorrect Options:** * **A. Proteins:** While immunoglobulins are made of amino acids, "Proteins" is too broad. In biochemistry, when a protein is conjugated with a non-protein group (carbohydrate), "Glycoprotein" is the more precise and correct classification. * **C. Proteoglycans:** These consist of a small core protein with very long, unbranched glycosaminoglycan (GAG) chains (e.g., heparin, chondroitin sulfate). In proteoglycans, the carbohydrate content is much higher (up to 95%) than the protein content, which is the opposite of immunoglobulins. * **D. Glycosides:** These are non-protein molecules where a sugar is bound to another functional group via a glycosidic bond (e.g., Cardiac glycosides like Digoxin). They do not have the complex polypeptide structure of antibodies. **High-Yield Facts for NEET-PG:** * **Site of Attachment:** Carbohydrates are usually **N-linked** to the Asparagine residues in the $C_H2$ domain of the IgG molecule. * **Most Abundant:** IgG is the most abundant immunoglobulin in serum. * **Acute Phase Reactants:** Most plasma proteins (like Transferrin, Ceruloplasmin, and Immunoglobulins) are glycoproteins, with the notable **exception of Albumin**, which is a simple protein.

Practice by Chapter

Hemoglobin Structure and Function

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Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve

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Hemoglobin Variants and Hemoglobinopathies

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Thalassemias

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Methemoglobin and Abnormal Hemoglobins

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Hemoglobin Synthesis

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Heme Synthesis and Porphyrias

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Iron Absorption and Transport

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Iron Storage and Recycling

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Disorders of Iron Metabolism

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Anemia: Biochemical Aspects

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Biochemistry of Hemostasis

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