Hemoglobin and Iron Metabolism Practice Questions

Q: In sickle cell trait, how many bands are typically found?

2. ### Explanation **1. Why Option A (2) is Correct:** Sickle cell trait (HbAS) is the heterozygous state where an individual inherits one normal $\beta$-globin gene and one mutated $\beta^S$ gene. On **Hemoglobin Electrophoresis** (alkaline pH), two distinct bands are visible: * **HbA:** Represents the normal adult hemoglobin (usually 50–60%). * **HbS:** Represents the sickle hemoglobin (usually 35–45%). Because both genes are expressed (codominance), both proteins are synthesized and separated based on their electrical charge. HbA moves faster toward the anode, while HbS moves slower due to the substitution of negatively charged glutamic acid with neutral valine at the 6th position of the $\beta$-chain. **2. Why Other Options are Incorrect:** * **Option B (1):** A single band is seen in normal adults (HbA) or those with homozygous Sickle Cell Disease (HbSS), where only HbS (and some HbF) is present. * **Option C & D (4 or 5):** These are incorrect as they do not correspond to the standard electrophoretic pattern of sickle cell trait. Multiple bands (3 or more) might be seen in complex compound heterozygous states (e.g., HbSC or HbS-$\beta$ Thalassemia) where HbF and HbA2 are also significantly elevated. **3. Clinical Pearls for NEET-PG:** * **Electrophoresis Mobility (Alkaline pH):** Remember the mnemonic **"A Fat Santa Claus"** (from fastest to slowest: Hb**A** > Hb**F** > Hb**S** > Hb**C**). * **HbS Mutation:** Point mutation (GAG $\rightarrow$ GTG) resulting in **Valine** replacing **Glutamic acid** at the 6th position of the $\beta$-globin chain. * **Sickle Cell Trait Protection:** Individuals with HbAS are naturally protected against severe *Plasmodium falciparum* malaria. * **Diagnosis:** In Sickle Cell Trait, HbA is always greater than HbS (HbA > HbS). If HbS > HbA, suspect $S\beta^+$ thalassemia.

Q: Red or brown discoloration of teeth is seen in which condition?

Porphyria. **Explanation:** **Correct Option: A. Porphyria** The discoloration of teeth in Porphyria, specifically **Congenital Erythropoietic Porphyria (Gunther’s disease)**, is known as **Erythrodontia**. This occurs due to the excessive accumulation and deposition of **uroporphyrin I and coproporphyrin I** in the calcium phosphate of the enamel and dentin during tooth formation. Under ultraviolet (Wood’s) light, these teeth exhibit a characteristic **reddish-pink fluorescence**. **Incorrect Options:** * **B. Internal Resorption:** This is a pathological process where the pulp tissue resorbs the dentin from within. It often results in a localized "Pink spot of Mummery," but it does not cause generalized red/brown discoloration of all teeth. * **C. Nasmyth’s Membrane:** Also known as the primary enamel cuticle, this is a thin residue of the enamel organ. It can sometimes take up extrinsic stains (green or orange) from food or bacteria in children but is not associated with intrinsic red/brown pathology. * **D. Silver Impregnation:** Exposure to silver (argyria) typically results in a **slate-grey or black** discoloration of tissues, not red or brown. **High-Yield Clinical Pearls for NEET-PG:** * **Gunther’s Disease (CEP):** Deficiency of **Uroporphyrinogen III synthase**. It is characterized by extreme photosensitivity, blistering, scarring, and erythrodontia. * **Tetracycline Staining:** Causes yellowish-brown discoloration; these teeth also fluoresce (bright yellow) under UV light. * **Fluorosis:** Causes "mottled enamel" with chalky white patches or brownish staining depending on the severity. * **Alkaptonuria:** While it causes dark urine (ochronosis), it typically stains cartilage and sclera, not primarily the teeth.

Q: What is the key regulatory enzyme in hepatic biosynthesis of heme?

ALA synthase. **Explanation:** **1. Why ALA Synthase is Correct:** Aminolevulinate synthase (**ALA synthase**) is the **rate-limiting and key regulatory enzyme** of heme biosynthesis in the liver. It catalyzes the condensation of Succinyl CoA and Glycine to form $\delta$-aminolevulinic acid (ALA), requiring **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. In the liver, this enzyme is strictly regulated by **heme** via feedback inhibition (repression of enzyme synthesis and inhibition of mitochondrial transport). **2. Why Other Options are Incorrect:** * **Decarboxylase (Uroporphyrinogen Decarboxylase):** This enzyme converts Uroporphyrinogen III to Coproporphyrinogen III. While its deficiency leads to *Porphyria Cutanea Tarda*, it is not the rate-limiting step. * **Coproporphyrinogen Oxidase:** This mitochondrial enzyme converts Coproporphyrinogen III to Protoporphyrinogen IX. Deficiency causes *Hereditary Coproporphyria*. * **Protoporphyrinogen Oxidase:** This enzyme converts Protoporphyrinogen IX to Protoporphyrin IX. Deficiency leads to *Variegate Porphyria*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoforms:** There are two isoforms: **ALAS1** (ubiquitous/hepatic) and **ALAS2** (erythroid-specific). ALAS2 is regulated by **iron** levels, not heme. * **Cofactor:** B6 deficiency can lead to **Sideroblastic Anemia** because ALA synthase cannot function. * **Inducers:** Drugs like barbiturates and griseofulvin induce ALAS1, which can precipitate acute attacks of porphyria. * **Inhibitors:** Hematin (a heme analog) is used to treat porphyria attacks because it inhibits ALA synthase.

Q: An increase in the concentration of 2,3-bisphosphoglycerate (2,3-DPG) may be seen in all of the following conditions except?

Hypoxanthine. **Explanation:** The concentration of **2,3-bisphosphoglycerate (2,3-BPG/DPG)** in erythrocytes is a critical regulator of hemoglobin’s oxygen affinity. It binds to the central cavity of the deoxyhemoglobin tetramer, stabilizing the **T (Tense) state** and shifting the oxygen dissociation curve to the **right**, facilitating oxygen unloading to tissues. **Why Hypoxanthine is the Correct Answer:** Hypoxanthine is a purine derivative and a breakdown product of adenosine monophosphate (AMP). It does not enter the glycolytic pathway or the **Rapoport-Luebering shunt** (the pathway responsible for 2,3-BPG synthesis). Therefore, it has no direct role in increasing 2,3-BPG levels. **Analysis of Incorrect Options:** * **Anemia & Hypoxia:** These conditions trigger a compensatory increase in 2,3-BPG. In anemia (low hemoglobin) and hypoxia (low arterial $PO_2$), the body increases 2,3-BPG to enhance oxygen delivery to peripheral tissues by decreasing hemoglobin's affinity for $O_2$. * **Inosine:** In blood banking, inosine is added to stored blood as a "rejuvenation" fluid. Inosine is a nucleoside that can be converted into **Ribose-5-phosphate**, which then enters the pentose phosphate pathway and subsequently the glycolytic pathway, eventually leading to the synthesis of 2,3-BPG. This helps restore the oxygen-carrying efficiency of stored blood. **Clinical Pearls for NEET-PG:** * **Rapoport-Luebering Shunt:** Uses the enzyme *2,3-BPG synthase/phosphatase* to bypass the phosphoglycerate kinase step of glycolysis. * **Stored Blood:** 2,3-BPG levels **decrease** in stored blood, causing a "left shift" (increased $O_2$ affinity). This is why inosine is added. * **Fetal Hemoglobin (HbF):** Has a lower affinity for 2,3-BPG compared to HbA, allowing HbF to have a higher affinity for $O_2$, facilitating oxygen transfer from mother to fetus. * **High Altitude:** Chronic exposure leads to an increase in 2,3-BPG as an adaptive mechanism.

Q: A polypeptide of hemoglobin closely resembles which of the following?

Myoglobin. **Explanation:** The correct answer is **Myoglobin (Option C)**. **Why Myoglobin is correct:** Hemoglobin is a tetrameric protein ($α_2β_2$), whereas myoglobin is a monomeric protein. Despite this difference in quaternary structure, the individual polypeptide chains (subunits) of hemoglobin are structurally very similar to myoglobin. Both share a common evolutionary origin and possess the **"globin fold"**—a characteristic tertiary structure consisting of eight α-helices (labeled A through H) that create a hydrophobic pocket to house the heme group. This structural homology allows both proteins to bind oxygen reversibly. **Why other options are incorrect:** * **Collagen:** A fibrous structural protein characterized by a triple helix (not α-helices) and a repeating Gly-X-Y amino acid sequence. It does not bind oxygen or contain heme. * **Albumin:** The most abundant plasma protein, primarily responsible for maintaining oncotic pressure and transporting various ligands (bilirubin, fatty acids, drugs). Its structure is globular but lacks the globin fold and heme moiety. * **Transferrin:** A plasma glycoprotein specifically designed to transport iron ($Fe^{3+}$) in the blood. While involved in iron metabolism, its structure and function are distinct from the oxygen-binding globin family. **High-Yield Clinical Pearls for NEET-PG:** * **Oxygen Dissociation Curve:** Myoglobin exhibits a **hyperbolic** curve (high affinity, storage function), while Hemoglobin exhibits a **sigmoidal** curve (cooperative binding, transport function). * **P50 Value:** The $P_{50}$ of myoglobin is much lower (~1–2 mmHg) than that of hemoglobin (~26 mmHg), reflecting myoglobin's higher affinity for $O_2$. * **Homology:** While the primary amino acid sequences of myoglobin and hemoglobin subunits differ significantly, their **tertiary structures** are nearly identical.

Q: Glucose is linked to haemoglobin through which type of linkage?

N-linkage. **Explanation:** The formation of glycated hemoglobin (HbA1c) is a non-enzymatic process known as **glycation**. This process involves the attachment of glucose to the hemoglobin molecule, specifically at the **N-terminal valine** of the beta-globin chain. 1. **Why N-linkage is correct:** The aldehyde group of glucose reacts with the free amino group (-NH₂) of the N-terminal valine. This initially forms an unstable aldimine (Schiff base), which then undergoes an **Amadori rearrangement** to form a stable ketoamine. Because the bond is formed with a nitrogen atom of the amino group, it is classified as an **N-linkage**. 2. **Why other options are incorrect:** * **O-linkage:** This typically refers to O-glycosylation, where sugars attach to the oxygen atom of hydroxyl groups (e.g., on Serine or Threonine residues). This is an enzymatic process common in glycoproteins, not HbA1c. * **C-C linkage:** Carbon-carbon bonds are extremely stable and do not form spontaneously between glucose and proteins under physiological conditions. * **O-H linkage:** This refers to a hydroxyl group or hydrogen bonding, not a covalent structural linkage between a sugar and a protein backbone. **Clinical Pearls for NEET-PG:** * **HbA1c reflects glycemic control** over the preceding **8–12 weeks** (the average lifespan of an RBC). * The reaction is **non-enzymatic**; its rate is directly proportional to the blood glucose concentration. * **False Lows:** Seen in conditions with high RBC turnover (e.g., Hemolytic anemia, recent hemorrhage, or treatment with Erythropoietin). * **False Highs:** Seen in Iron deficiency anemia (due to increased RBC lifespan and altered glycation rates).

Q: Which of the following proteins inhibits heme loss from plasma?

Hemopexin. **Explanation:** The correct answer is **Hemopexin**. **1. Why Hemopexin is correct:** Hemopexin is a plasma glycoprotein synthesized by the liver with the highest known affinity for **heme**. When intravascular hemolysis occurs, hemoglobin is released and quickly dissociates into globin and heme. While **Haptoglobin** binds to free hemoglobin, **Hemopexin** specifically binds to the free heme (ferriheme). This binding prevents heme-mediated oxidative damage to tissues and, crucially, prevents the loss of iron via urinary excretion. The heme-hemopexin complex is then transported to the liver, where it is internalized via receptor-mediated endocytosis (CD91 receptor). **2. Why the other options are incorrect:** * **Ferritin:** This is the primary **intracellular** storage form of iron found in the liver, spleen, and bone marrow. While small amounts circulate in the plasma (reflecting total body iron stores), its role is storage, not heme transport. * **Ceruloplasmin:** This is a ferroxidase enzyme containing copper. Its primary role is to oxidize ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$) so it can bind to transferrin. It does not bind heme. * **Hemosiderin:** This is an insoluble iron-storage complex, usually derived from the partial digestion of ferritin. It is found within cells (macrophages) during states of iron overload, not as a plasma transport protein. **Clinical Pearls for NEET-PG:** * **Haptoglobin vs. Hemopexin:** Haptoglobin is the first line of defense (binds Hb). Once haptoglobin is saturated, free heme is released and bound by Hemopexin. * **Hemolysis Marker:** Low serum levels of Haptoglobin and Hemopexin are diagnostic markers for **intravascular hemolysis**. * **Transferrin:** Remember that Transferrin transports **ionic iron**, not heme or hemoglobin.

Q: Which of the following is given with iron to increase its absorption?

Citrus fruits. **Explanation:** Iron absorption is a highly regulated process occurring primarily in the duodenum and upper jejunum. The correct answer is **Citrus fruits** because they are rich in **Vitamin C (Ascorbic acid)**. **Why Citrus Fruits are Correct:** Iron exists in two forms: Ferric ($Fe^{3+}$) and Ferrous ($Fe^{2+}$). Only the **Ferrous ($Fe^{2+}$)** form is easily absorbed by the divalent metal transporter 1 (DMT1) in the intestinal mucosa. Vitamin C acts as a reducing agent, converting ferric iron to the more soluble ferrous form. Additionally, it forms a soluble iron-ascorbate complex that prevents iron from precipitating in the alkaline environment of the small intestine. **Why Other Options are Incorrect:** * **Milk:** Contains calcium and phosphates. Calcium competes with iron for transport pathways, and phosphates can form insoluble complexes with iron, significantly inhibiting absorption. * **Antacids & Alkalies:** Iron requires an acidic gastric environment to remain in its soluble ferrous state. Antacids and alkalies neutralize gastric acid, promoting the conversion of iron into insoluble ferric hydroxides, which are poorly absorbed. **NEET-PG High-Yield Pearls:** * **Promoters of Iron Absorption:** Vitamin C (Ascorbic acid), Gastric Acid (HCl), and Amino acids (Cysteine). * **Inhibitors of Iron Absorption:** Phytates (cereals), Oxalates (spinach), Tannins (tea/coffee), Calcium (milk), and Tetracyclines. * **Clinical Tip:** Patients prescribed oral iron supplements are advised to take them with orange juice and avoid tea or milk for at least 2 hours to maximize bioavailability.

Q: What is the bilirubin binding capacity for the high-affinity site of serum albumin?

25 mg bilirubin/100 ml of plasma. **Explanation:** The transport of unconjugated bilirubin in the blood is critical because it is hydrophobic and potentially neurotoxic. Bilirubin binds to serum albumin in a reversible manner. Albumin possesses two types of binding sites for bilirubin: 1. **High-affinity site:** One molecule of albumin binds one molecule of bilirubin very tightly. 2. **Low-affinity site:** Additional bilirubin molecules bind weakly when the primary site is saturated. In a healthy adult, the **high-affinity binding capacity** is approximately **25 mg of bilirubin per 100 ml of plasma**. When serum bilirubin levels exceed this threshold (hyperbilirubinemia), the albumin becomes saturated. The "free" or unbound bilirubin can then cross the blood-brain barrier, leading to neurotoxicity (Kernicterus). **Analysis of Options:** * **Option B (Correct):** 25 mg/dL represents the physiological limit of the primary high-affinity binding site of albumin. * **Options A, C, and D:** These values are incorrect. 15 mg/dL is often cited as a "danger zone" in neonatal jaundice where phototherapy or exchange transfusion is considered, but it does not represent the maximum binding capacity. 35 and 45 mg/dL far exceed the safe binding capacity of albumin. **High-Yield Clinical Pearls for NEET-PG:** * **Kernicterus:** Occurs when unconjugated bilirubin exceeds albumin-binding capacity (>20–25 mg/dL in full-term infants). * **Drug Interactions:** Drugs like **Sulfonamides, Salicylates, and Phenytoin** can displace bilirubin from albumin binding sites, increasing the risk of Kernicterus even at lower total bilirubin levels. * **Acidosis:** Decreases the affinity of albumin for bilirubin, further increasing the fraction of free bilirubin. * **Bilirubin-Albumin Complex:** This complex is too large to be filtered by the glomerulus; hence, unconjugated bilirubin never appears in the urine (acholuric jaundice).

Q: Mucosal transfer of iron in the gastrointestinal tract is mediated by which protein?

Apoferritin. **Explanation:** The absorption of iron in the gastrointestinal tract is a highly regulated process. Iron enters the enterocyte (mucosal cell) via the **Divalent Metal Transporter 1 (DMT-1)**. Once inside the cell, iron must be transported across the cytoplasm to the basolateral membrane for systemic absorption. This mucosal transfer is mediated by **Apoferritin**. Apoferritin binds with free ferrous iron ($Fe^{2+}$) to form **Ferritin**. This complex acts as a temporary storage form within the mucosal cell. If the body requires iron, it is released and transported into the blood; if not, the iron remains trapped as ferritin and is lost when the mucosal cell is sloughed off (the "mucosal block" phenomenon). **Analysis of Options:** * **A. Transferrin:** This is the primary protein responsible for transporting iron in the **plasma**, not within the mucosal cell. * **C. Apotransferrin:** This is the iron-free form of transferrin found in the blood. It becomes transferrin once it binds two molecules of ferric iron ($Fe^{3+}$). * **D. Ferritin:** While ferritin is the storage form, **Apoferritin** is the specific protein shell that mediates the binding and transfer process within the mucosa. **High-Yield Clinical Pearls for NEET-PG:** * **Hepcidin:** The master regulator of iron metabolism; it inhibits **Ferroportin**, preventing iron exit from enterocytes into the blood. * **Hephaestin:** A ferroxidase that converts $Fe^{2+}$ to $Fe^{3+}$ at the basolateral membrane to allow binding to plasma transferrin. * **Vitamin C:** Enhances iron absorption by maintaining iron in the more soluble ferrous ($Fe^{2+}$) state.

Question 1

In sickle cell trait, how many bands are typically found?

Accepted Answer

2

Answer

1

Answer

4

Answer

5

Question 2

Red or brown discoloration of teeth is seen in which condition?

Accepted Answer

Porphyria

Answer

Internal resorption

Answer

Nasmyth's membrane

Answer

Silver impregnation

Question 3

What is the key regulatory enzyme in hepatic biosynthesis of heme?

Accepted Answer

ALA synthase

Answer

Decarboxylase

Answer

Coproporphyrinogen oxidase

Answer

Protoporphyrinogen oxidase

Question 4

An increase in the concentration of 2,3-bisphosphoglycerate (2,3-DPG) may be seen in all of the following conditions except?

Accepted Answer

Hypoxanthine

Answer

Anemia

Answer

Hypoxia

Answer

Inosine

Question 5

A polypeptide of hemoglobin closely resembles which of the following?

Accepted Answer

Myoglobin

Answer

Collagen

Answer

Albumin

Answer

Transferrin

Question 6

Glucose is linked to haemoglobin through which type of linkage?

Accepted Answer

N-linkage

Answer

O-linkage

Answer

C-C linkage

Answer

O-H linkage

Question 7

Which of the following proteins inhibits heme loss from plasma?

Accepted Answer

Hemopexin

Answer

Ferritin

Answer

Ceruloplasmin

Answer

Hemosiderin

Question 8

Which of the following is given with iron to increase its absorption?

Accepted Answer

Citrus fruits

Answer

Milk

Answer

Antacids

Answer

Alkalies

Question 9

What is the bilirubin binding capacity for the high-affinity site of serum albumin?

Accepted Answer

25 mg bilirubin/100 ml of plasma

Answer

15 mg bilirubin/100 ml of plasma

Answer

35 mg bilirubin/100 ml of plasma

Answer

45 mg bilirubin/100 ml of plasma

Question 10

Mucosal transfer of iron in the gastrointestinal tract is mediated by which protein?

Accepted Answer

Apoferritin

Answer

Transferrin

Answer

Apotransferrin

Answer

Ferritin

Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism — MCQs

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