Hemoglobin and Iron Metabolism — MCQs

A 34-year-old female has a history of intermittent episodes of severe abdominal pain. She has had multiple abdominal surgeries and exploratory procedures with no abnormal findings. Her urine appears dark during an attack and gets even darker if exposed to sunlight. The attacks seem to peak after she takes erythromycin, because of her penicillin allergy. This patient most likely has difficulty in synthesizing which one of the following?

Q198

A patient who was in excruciating pain all over his body was taken to the hospital. In recent years, he has experienced these episodes frequently. When exercising vigorously, the pain begins. Anemia was detected on blood examination along with sickled RBCs as opposed to normal biconcave ones. It was determined that he had sickle cell anemia. What substitution takes place in sickle cell anemia?

Q199

Which of the following correctly represents the effect of the mutation causing sickle cell anemia?

Q200

Hemoglobin with iron in ferric form is

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 191: Haemoglobin can bind to all of the following except?

A. O2
B. CO2
C. SO2 (Correct Answer)
D. NO

Explanation: **Explanation:** Hemoglobin (Hb) is a complex globular protein designed to transport respiratory gases and signaling molecules. The correct answer is **SO₂ (Sulfur Dioxide)** because it does not form a stable chemical bond with hemoglobin in the human body; instead, it is highly soluble in the upper respiratory tract and converts to sulfurous acid upon contact with moisture. **Why the other options are incorrect:** * **O₂ (Oxygen):** Hb binds oxygen cooperatively to the heme iron ($Fe^{2+}$) to form **oxyhemoglobin**. This is its primary physiological function. * **CO₂ (Carbon Dioxide):** Hb transports approximately 15-20% of $CO_2$ as **carbaminohemoglobin**. Crucially, $CO_2$ binds to the **amino-terminal groups** of the globin chains, not the heme iron. * **NO (Nitric Oxide):** Hb acts as a scavenger for NO. It can bind to the heme iron or to specific cysteine residues (forming **S-nitrosothiol**) to assist in vasodilation and blood pressure regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Carbon Monoxide (CO):** Binds to heme iron with an affinity **210–250 times greater** than oxygen, forming carboxyhemoglobin and shifting the oxygen dissociation curve to the **left**. * **2,3-BPG:** Binds to the central cavity of the hemoglobin tetramer (specifically to beta chains), stabilizing the **T-state (Tense)** and promoting oxygen unloading (Right shift). * **Methemoglobin:** Occurs when iron is oxidized to the **ferric state ($Fe^{3+}$)**; it cannot bind $O_2$. * **Sulfhemoglobin:** While "Sulf-" sounds like $SO_2$, it actually refers to the incorporation of a sulfur atom into the porphyrin ring (often due to drugs like sulfonamides). It is irreversible and reduces $O_2$ affinity.

Question 192: What is the daily loss of iron per day in a healthy adult male?

A. 0.06 mg
B. 0.6 mg (Correct Answer)
C. 60 mg
D. 600 mg

Explanation: **Explanation:** The correct answer is **0.6 mg (Option B)**. In a healthy adult male, iron balance is maintained through a very tight regulatory system because the human body lacks a physiological mechanism for active iron excretion. Iron is lost primarily through the **desquamation (shedding) of epithelial cells** from the skin and gastrointestinal tract, as well as minute amounts in bile, sweat, and urine. On average, a healthy male loses approximately **0.6 to 1.0 mg of iron per day**. To maintain balance, an equivalent amount must be absorbed from the diet. Since only about 10% of dietary iron is absorbed, a daily intake of 10 mg is typically recommended. **Analysis of Incorrect Options:** * **Option A (0.06 mg):** This value is far too low to account for the constant cellular turnover of the gut and skin. * **Option C (60 mg):** This is an excessive amount. Total body iron is only about 3–4 grams; losing 60 mg daily would lead to rapid, life-threatening anemia. * **Option D (600 mg):** This value exceeds the total amount of iron stored as ferritin in many individuals. **NEET-PG High-Yield Pearls:** * **Menstruating Females:** Daily iron loss is higher, averaging **1.3 to 1.5 mg/day** due to menstrual blood loss. * **Absorption Site:** Iron is primarily absorbed in the **duodenum** and upper jejunum. * **Hepcidin:** This is the "master regulator" of iron metabolism; it inhibits iron release by binding to **ferroportin**. * **Storage:** Iron is stored as **ferritin** (soluble) and **hemosiderin** (insoluble). Ferritin levels are the most sensitive lab index for diagnosing early iron deficiency anemia.

Question 193: Which of the following proteins binds with free hemoglobin in the plasma?

A. Albumin
B. Haptoglobin (Correct Answer)
C. Pre-albumin
D. Ceruloplasmin

Explanation: ### Explanation **Correct Answer: B. Haptoglobin** **Mechanism and Concept:** When red blood cells undergo **intravascular hemolysis**, free hemoglobin (Hb) is released into the plasma. Free Hb is toxic as it can cause oxidative tissue damage and is small enough to be filtered by the renal glomeruli, leading to kidney injury (hemoglobinuria). **Haptoglobin** is an acute-phase reactant synthesized by the liver that specifically binds to free hemoglobin dimers to form a large **Hb-Haptoglobin complex**. This complex is too large to be filtered by the kidneys and is instead rapidly cleared by the reticuloendothelial system (specifically via CD163 receptors on macrophages). This mechanism conserves iron and protects the kidneys. **Why other options are incorrect:** * **A. Albumin:** While albumin is a non-specific transport protein, it primarily binds to **methemalbumin** (heme + albumin) only after haptoglobin and hemopexin are saturated. * **C. Pre-albumin (Transthyretin):** This protein is responsible for the transport of thyroxine (T4) and retinol-binding protein; it has no role in hemoglobin binding. * **D. Ceruloplasmin:** This is a copper-binding protein with ferroxidase activity (converting $Fe^{2+}$ to $Fe^{3+}$); it is essential for iron mobilization but does not bind hemoglobin. **High-Yield Clinical Pearls for NEET-PG:** * **Marker of Hemolysis:** A **decreased serum haptoglobin level** is the most sensitive laboratory indicator of intravascular hemolysis (because it is consumed while clearing Hb). * **Hemopexin:** If haptoglobin is exhausted, **Hemopexin** acts as the secondary backup to bind free **heme** (not whole hemoglobin). * **Acute Phase Reactant:** Since haptoglobin increases during inflammation, its levels may appear "normal" during hemolysis if a concurrent inflammatory state exists.

Question 194: Hemoglobin does not bind with:

A. Oxygen
B. Carbon dioxide
C. Carbon monoxide
D. HCN (Correct Answer)

Explanation: **Explanation:** The correct answer is **HCN (Hydrogen Cyanide)**. Hemoglobin (Hb) is a globular protein designed to transport gases by binding to the ferrous iron ($Fe^{2+}$) in its heme group or to specific amino acid residues. **1. Why HCN is the correct answer:** Hydrogen cyanide (HCN) does not bind to hemoglobin. Instead, the cyanide ion ($CN^-$) has a high affinity for the **ferric iron ($Fe^{3+}$)** found in **cytochrome oxidase a3** (part of the mitochondrial electron transport chain). By inhibiting this enzyme, cyanide halts cellular respiration, leading to "histotoxic hypoxia." While cyanide can bind to **methemoglobin** (which contains $Fe^{3+}$), it does not bind to normal functional hemoglobin ($Fe^{2+}$). **2. Why the other options are incorrect:** * **Oxygen (A):** Hemoglobin’s primary function is the reversible binding of $O_2$ to the $Fe^{2+}$ of heme to form oxyhemoglobin. * **Carbon dioxide (B):** $CO_2$ binds to the N-terminal amino groups of the globin chains (not the heme) to form **carbaminohemoglobin**. This accounts for about 15-20% of $CO_2$ transport. * **Carbon monoxide (C):** CO binds to the $Fe^{2+}$ of heme with an affinity **200–250 times greater** than oxygen, forming carboxyhemoglobin and causing CO poisoning. **Clinical Pearls for NEET-PG:** * **Cyanide Poisoning Treatment:** We intentionally induce methemoglobinemia (using nitrites) because methemoglobin ($Fe^{3+}$) acts as a "sink" to pull cyanide away from cytochrome oxidase. * **Bohr Effect:** High $CO_2$ and low pH decrease Hb's affinity for $O_2$, shifting the dissociation curve to the **right**. * **2,3-BPG:** An important regulator that stabilizes the "T" (Tense) state of Hb, promoting oxygen unloading.

Question 195: Along with succinyl CoA, which of the following amino acids serves as a starting material in heme synthesis?

A. Lysine
B. Leucine
C. Glycine (Correct Answer)
D. Alanine

Explanation: ### Explanation **Correct Option: C (Glycine)** Heme synthesis begins in the mitochondria with the condensation of **Succinyl CoA** (from the TCA cycle) and the amino acid **Glycine**. This reaction is catalyzed by the enzyme **ALA Synthase (ALAS)**, which requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. This is the **rate-limiting and committed step** of heme biosynthesis. Together, they form $\delta$-aminolevulinic acid (ALA). Glycine is the only amino acid that provides the carbon and nitrogen atoms required for the porphyrin ring structure. **Why Incorrect Options are Wrong:** * **A (Lysine):** An essential, purely ketogenic amino acid. It is involved in carnitine synthesis and collagen cross-linking but plays no role in the porphyrin pathway. * **B (Leucine):** A branched-chain amino acid (BCAA) and purely ketogenic. It is primarily used for protein synthesis and energy metabolism. * **D (Alanine):** A key glucogenic amino acid involved in the Cahill cycle (glucose-alanine cycle) for nitrogen transport from muscle to liver, but it is not a substrate for heme. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** ALA Synthase (ALAS1 in liver; ALAS2 in erythroid cells). * **Cofactor Alert:** Deficiency of **Vitamin B6** leads to Sideroblastic Anemia because ALA synthase cannot function, causing iron to accumulate in mitochondria (forming ringed sideroblasts). * **Inhibitors:** Heme acts as a feedback inhibitor of ALAS. Lead poisoning inhibits the subsequent enzymes (ALA Dehydratase and Ferrochelatase). * **Location:** Heme synthesis occurs partly in the **mitochondria** (first and last three steps) and partly in the **cytosol**. Memory aid: "The first and the last are in the furnace (mitochondria)."

Question 196: Iron absorption is inhibited by all except:

A. Vitamin C (Correct Answer)
B. Phytates
C. Caffeine
D. Milk

Explanation: **Explanation:** Iron absorption occurs primarily in the **duodenum and upper jejunum**. Dietary iron exists in two forms: Heme (animal sources) and Non-heme (plant sources). The absorption of non-heme iron is highly sensitive to dietary enhancers and inhibitors. **Why Vitamin C is the Correct Answer:** Vitamin C (Ascorbic acid) is a potent **enhancer** of iron absorption, not an inhibitor. It facilitates absorption through two mechanisms: 1. **Reduction:** It reduces ferric iron ($Fe^{3+}$) to the more soluble ferrous form ($Fe^{2+}$), which is the only form transported across the apical membrane by the Divalent Metal Transporter 1 (DMT1). 2. **Chelation:** It forms a soluble iron-ascorbate complex that prevents iron from precipitating in the alkaline environment of the small intestine. **Why the Other Options are Wrong (Inhibitors):** * **Phytates:** Found in whole grains and legumes, they bind to iron to form insoluble complexes, preventing absorption. * **Caffeine/Tannins:** Found in tea and coffee, these polyphenols bind iron and significantly reduce its bioavailability. * **Milk (Calcium/Phosphates):** Calcium is a unique inhibitor that interferes with both heme and non-heme iron absorption. Phosphates and casein in milk also form insoluble precipitates with iron. **High-Yield Clinical Pearls for NEET-PG:** * **Hepcidin:** The master regulator of iron homeostasis; it inhibits iron absorption by degrading **Ferroportin**. * **Achlorhydria:** Gastric acid is essential for iron solubilization; hence, PPI use or gastrectomy leads to Iron Deficiency Anemia (IDA). * **Meat Factor:** Animal proteins enhance non-heme iron absorption (similar to Vitamin C). * **Storage:** Iron is stored as **Ferritin** (labile) and **Hemosiderin** (stable/insoluble).

Question 197: A 34-year-old female has a history of intermittent episodes of severe abdominal pain. She has had multiple abdominal surgeries and exploratory procedures with no abnormal findings. Her urine appears dark during an attack and gets even darker if exposed to sunlight. The attacks seem to peak after she takes erythromycin, because of her penicillin allergy. This patient most likely has difficulty in synthesizing which one of the following?

A. Heme (Correct Answer)
B. Creatine phosphate
C. Cysteine
D. Thymine

Explanation: **Explanation:** The clinical presentation describes a classic case of **Acute Intermittent Porphyria (AIP)**. AIP is an autosomal dominant disorder caused by a deficiency of the enzyme **Porphobilinogen (PBG) deaminase**, a key enzyme in the **Heme synthesis** pathway. **Why Heme is the Correct Answer:** 1. **Clinical Triad:** The patient presents with the "3 Ps": **P**ainful abdomen (neurovisceral symptoms leading to unnecessary surgeries), **P**ort-wine colored urine (due to oxidation of PBG to porphobilin), and **P**sychological disturbances (though not mentioned here, common in AIP). 2. **Urine Findings:** PBG and ALA (delta-aminolevulinic acid) accumulate in the urine. Upon exposure to light/air, PBG polymerizes into porphobilin, causing the urine to darken. 3. **Precipitating Factors:** Attacks are triggered by drugs that induce **Cytochrome P450** (like Erythromycin), alcohol, or fasting. These triggers increase the demand for heme, upregulating ALA synthase and leading to the accumulation of toxic intermediates. **Why Other Options are Wrong:** * **Creatine phosphate:** Synthesized from glycine, arginine, and methionine; not related to porphyrin metabolism. * **Cysteine:** A non-essential amino acid synthesized from methionine and serine; its deficiency causes homocystinuria, not abdominal pain or dark urine. * **Thymine:** A pyrimidine base; defects in its synthesis or salvage lead to conditions like Orotic Aciduria (presenting with megaloblastic anemia and growth failure). **NEET-PG High-Yield Pearls:** * **Enzyme Deficient in AIP:** PBG Deaminase (also known as HMB Synthase). * **Key Feature:** AIP is a "non-photosensitive" porphyria (unlike Porphyria Cutanea Tarda) because the accumulation occurs *before* the formation of porphyrin rings. * **Management:** Treatment involves **Hematin/Heme arginate** and **Glucose** infusion, which inhibit ALA synthase (the rate-limiting enzyme) via feedback inhibition.

Question 198: A patient who was in excruciating pain all over his body was taken to the hospital. In recent years, he has experienced these episodes frequently. When exercising vigorously, the pain begins. Anemia was detected on blood examination along with sickled RBCs as opposed to normal biconcave ones. It was determined that he had sickle cell anemia. What substitution takes place in sickle cell anemia?

A. Substitution of glutamic acid by valine at the 6th position (Correct Answer)
B. Substitution of valine by glutamic acid at the 5th position
C. Substitution of glutamic acid by valine at the 5th position
D. Substitution of valine by glutamic acid at the 6th position

Explanation: ***Substitution of glutamic acid by valine at the 6th position***- This is the defining molecular defect in **sickle cell anemia** (HbS), resulting from a point mutation (GAG $\rightarrow$ GTG) in the $\beta$-globin gene.- The replacement of the hydrophilic amino acid **glutamic acid** by the hydrophobic amino acid **valine** at the **6th position** facilitates the polymerization of **deoxy-HbS** under low oxygen tension, causing RBC sickling.*Substitution of valine by glutamic acid at the 6th position*- This change is the **opposite** of the mutation causing HbS; HbS involves the substitution of a hydrophilic residue (Glutamic acid) with a **hydrophobic** one (Valine).- This particular reverse substitution would not lead to the formation of the **deoxy-HbS polymers** responsible for the sickling phenomenon.*Substitution of valine by glutamic acid at the 5th position*- The amino acid at the 5th position of the $\beta$-globin chain is typically **proline**, and the crucial molecular abnormality in HbS involves the **6th position**, not the 5th.- The defining mutation involves Glutamic acid being replaced by Valine, not Valine being replaced by Glutamic acid.*Substitution of glutamic acid by valine at the 5th position*- While the amino acid change (Glutamic acid $\rightarrow$ Valine) is correct for the type of substitution, the characteristic mutation of **sickle cell anemia** occurs specifically at the **6th position**.- Substitutions at different positions (like the 5th) usually result in other, distinct hemoglobin variants.

Question 199: Which of the following correctly represents the effect of the mutation causing sickle cell anemia?

A. Glutamate by valine at the 5th position
B. Valine by glutamate at the 5th position
C. Valine by glutamate at the 6th position
D. Glutamate by valine at the 6th position (Correct Answer)

Explanation: ***Glutamate by valine at the 6th position***- This mutation involves a single nucleotide substitution (A to T) in the $\beta$-globin gene, resulting in the replacement of hydrophilic **glutamate** with hydrophobic **valine** at the sixth position.- This change (E6V) creates a sticky patch on the **hemoglobin S (HbS)** molecule, leading to polymerization and sickling of red blood cells under conditions of low oxygen tension.*Valine by glutamate at the 6th position*- This represents the reverse substitution: replacing **valine** with **glutamate** at the 6th position.- This reversal of the substitution direction would describe the change from **HbS** back to the normal $\beta$-globin chain structure (**HbA**), not the cause of sickle cell anemia.*Glutamate by valine at the 5th position*- Although the amino acid substitution (**Glutamate** replaced by **Valine**) is correct, the position specified is inaccurate.- The critical substitution causing **sickle cell disease** is precisely at the **6th position** of the $\beta$-globin chain, not the 5th.*Valine by glutamate at the 5th position*- This option fails on two counts: the substitution direction is reversed (Valine $\rightarrow$ Glutamate), and the position of the mutation (**5th**) is incorrect.- The pathogenesis of sickle cell anemia depends on the replacement of the charged hydrophilic amino acid (**glutamate**) with the uncharged hydrophobic amino acid (**valine**) at position 6.

Question 200: Hemoglobin with iron in ferric form is

A. Methemoglobin (Correct Answer)
B. HbA
C. Fetal hemoglobin
D. HbS

Explanation: ***Methemoglobin***- **Methemoglobin** is characterized by the presence of iron in the **ferric state ($\text{Fe}^{3+}$)** in the heme group, which **cannot reversibly bind oxygen**, unlike the ferrous form. - This oxidized form of hemoglobin cannot function as an **oxygen carrier** and its accumulation leads to clinical conditions like **methemoglobinemia**. *HbA*- **HbA** (Adult hemoglobin) is the predominant form of hemoglobin in adults, and its iron must be in the **ferrous state ($\text{Fe}^{2+}$)** to bind and transport oxygen effectively.- The conversion of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ in normal HbA is constantly prevented by the **methemoglobin reductase system** (especially **NADH-cytochrome $b_5$ reductase**). *Fetal hemoglobin*- **Fetal hemoglobin (HbF)**, like HbA, uses iron in the **ferrous state ($\text{Fe}^{2+}$)** for reversible oxygen binding, reflecting its normal physiological function in the fetus. - HbF's unique feature is its structure ($\text{alpha}_2\text{gamma}_2$) which confers a **higher affinity for oxygen** by binding **2,3-BPG** less avidly than HbA, but this does not involve ferric iron. *HbS*- **HbS** (Sickle hemoglobin) is defective due to a mutation in the **beta-globin chain** (E6V), but the iron in its heme group remains in the **functional ferrous state ($\text{Fe}^{2+}$)**.- The abnormality in HbS relates to its **polymerization** upon deoxygenation, causing red blood cell sickling, not an oxidation state change of the heme iron.

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