Hemoglobin and Iron Metabolism Practice Questions

Q: A 10-year-old boy presented with muscle weakness and fatigue with increased lead in the blood. Which enzyme production in the liver is increased?

ALA synthase. **Explanation:** The correct answer is **ALA synthase**. This question tests the understanding of the heme biosynthetic pathway and the mechanism of lead poisoning. **Why ALA Synthase is correct:** Lead poisoning (Plumbism) primarily inhibits two enzymes in the heme synthesis pathway: **ALA dehydratase** (also known as Porphobilinogen synthase) and **Ferrochelatase**. When ALA dehydratase is inhibited, the production of heme decreases. Heme normally acts as a feedback inhibitor of **ALA synthase (ALAS-1)**, the rate-limiting enzyme of the pathway. Therefore, the deficiency of heme leads to the **derepression (upregulation)** of ALA synthase, causing an increase in its production and a subsequent accumulation of delta-aminolevulinic acid (ALA) in the blood and urine. **Why the other options are incorrect:** * **Ferrochelatase:** This enzyme is directly **inhibited** by lead, not increased. Its inhibition leads to the accumulation of Protoporphyrin IX (often measured as Zinc Protoporphyrin). * **Heme oxygenase:** This is the rate-limiting enzyme of **heme degradation** (converting heme to biliverdin). It is not the primary enzyme induced in response to lead-induced heme deficiency. * **Porphobilinogen deaminase:** This enzyme is involved in Acute Intermittent Porphyria. It is not specifically increased or inhibited by lead. **Clinical Pearls for NEET-PG:** * **Lead Poisoning Markers:** Increased urinary **delta-ALA** and increased **Zinc Protoporphyrin** (ZPP) in RBCs. * **Basophilic Stippling:** A classic peripheral smear finding in lead poisoning due to inhibition of pyrimidine 5'-nucleotidase. * **Clinical Signs:** "ABCDEF" – **A**nemia (Sideroblastic), **B**urton lines (gingival), **C**olic, **D**emyelination (wrist/foot drop), **E**ncephalopathy, **F**ree erythrocyte protoporphyrin. * **Antidotes:** Succimer (oral, preferred in children), CaNa₂EDTA, and Dimercaprol (BAL).

Q: Hemoglobin electrophoresis in SC A (sickle cell anemia) shows the presence of which hemoglobin?

HbA. **Explanation:** In **Sickle Cell Anemia (HbSS)**, there is a point mutation in the $\beta$-globin gene (glutamate replaced by valine at the 6th position). This results in the production of abnormal **HbS** ($\alpha_2\beta^s_2$). 1. **Why HbA is the correct answer (in the context of the question):** In a patient with homozygous Sickle Cell Anemia (HbSS), **HbA is completely absent** because there are no normal $\beta$-globin chains produced to form $\alpha_2\beta_2$. Therefore, on electrophoresis, the HbA band will show 0%. The question asks which hemoglobin is present; since HbA is the only one *not* present, it is often the focus of "except" style questions or identifying the disease state. *Note: In Sickle Cell Trait (HbAS), HbA is present (approx. 55-60%).* 2. **Analysis of Incorrect Options:** * **HbF (Fetal Hemoglobin):** In HbSS, HbF levels are usually **elevated** (5–15%) as a compensatory mechanism to inhibit the polymerization of HbS. * **HbA2:** This is typically **normal or slightly increased** (2–4%) in sickle cell patients. * **HbS:** Though not listed as an option, HbS is the predominant hemoglobin (85–95%) found on electrophoresis in these patients. **High-Yield Clinical Pearls for NEET-PG:** * **Electrophoresis Pattern:** On alkaline electrophoresis (pH 8.6), the order of migration from cathode (-) to anode (+) is **C $\rightarrow$ S $\rightarrow$ F $\rightarrow$ A** (Mnemonic: **C**ats **S**leep **F**ast **A**nywhere). * **Diagnosis:** The gold standard for diagnosis is **High-Performance Liquid Chromatography (HPLC)**. * **Management:** **Hydroxyurea** is used in treatment because it increases the concentration of **HbF**, which prevents sickling. * **Sickling Test:** Uses reducing agents like Sodium metabisulfite to induce sickling in vitro.

Q: A missense gene mutation in the beta-globin chain is characteristic of which condition?

Sickle cell anemia. ### Explanation **Correct Answer: B. Sickle cell anemia** **Why it is correct:** Sickle cell anemia is the classic example of a **missense mutation** (a type of point mutation). Specifically, there is a substitution of **Adenine by Thymine (GAG → GTG)** at the 6th codon of the beta-globin gene. This results in the replacement of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic). This single amino acid change causes Hemoglobin S (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Why the other options are incorrect:** * **A. Thalassemia:** Unlike sickle cell anemia (a qualitative defect), Thalassemia is a **quantitative defect**. It is most commonly caused by **splice-site mutations** or nonsense mutations (in $\beta$-thalassemia) and **large gene deletions** (in $\alpha$-thalassemia), leading to reduced or absent synthesis of globin chains. * **C. Hemoglobin Bart's:** This occurs in severe $\alpha$-thalassemia (hydrops fetalis) where all four $\alpha$-globin genes are deleted. It consists of **gamma-chain tetramers ($\gamma_4$)**. * **D. Hemoglobin H:** This occurs when three $\alpha$-globin genes are deleted. It consists of **beta-chain tetramers ($\beta_4$)**. Both Hb Bart's and HbH are results of gene deletions, not missense mutations. **NEET-PG High-Yield Pearls:** * **Mutation Type:** Transversion (Purine A to Pyrimidine T). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because Valine is less negatively charged than Glutamic acid. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Metabolic Trigger:** Acidosis, dehydration, and hypoxia shift the oxygen dissociation curve to the right, promoting sickling.

Q: Iron in heme is linked to globin through which amino acid residue?

Histidine. **Explanation:** The correct answer is **Histidine**. In the hemoglobin molecule, iron ($Fe^{2+}$) exists in a hexacoordinated state. Four of these coordination bonds are formed with the nitrogen atoms of the **porphyrin ring**. The fifth coordination bond is formed with the imidazole ring of a specific **Histidine** residue, known as the **Proximal Histidine (F8)**. * **Proximal Histidine (F8):** Directly binds to the iron atom, anchoring the heme group to the globin chain. * **Distal Histidine (E7):** Does not bind directly to iron but stabilizes the oxygen-binding site and prevents the oxidation of $Fe^{2+}$ to $Fe^{3+}$ (methemoglobin). **Why other options are incorrect:** * **Alanine & Glycine:** These are small, non-polar/neutral amino acids. They lack the necessary side-chain functional groups (like the imidazole ring) required to coordinate with a metal ion like iron in the heme pocket. * **Cysteine:** While cysteine can coordinate with metals (as seen in Zinc-finger proteins or Cytochrome P450), it is not the residue that links heme to globin in hemoglobin or myoglobin. **High-Yield Clinical Pearls for NEET-PG:** 1. **T-state vs. R-state:** When oxygen binds to the 6th coordination site, it pulls the iron into the plane of the porphyrin ring. This movement pulls the Proximal Histidine, triggering a conformational change from the **T (Tense)** state to the **R (Relaxed)** state. 2. **Methemoglobinemia:** If the iron is oxidized to the ferric state ($Fe^{3+}$), it cannot bind oxygen. The Distal Histidine plays a crucial role in preventing this. 3. **Carbon Monoxide (CO) Binding:** CO has a much higher affinity for heme than $O_2$. The Distal Histidine creates steric hindrance that forces CO to bind at an angle, slightly reducing its affinity and allowing for life-sustaining $O_2$ transport.

Q: Pale lemon yellow color of urine is due to all except?

Uroerythrin. The color of normal urine is primarily determined by the concentration of specific bile pigment derivatives. **Explanation of the Correct Answer:** **C. Uroerythrin** is the correct answer because it does not contribute to the pale lemon yellow color. Instead, uroerythrin is a pink or reddish pigment. It has a high affinity for uric acid crystals; when urine is cooled, uroerythrin precipitates with urates (forming "brick dust" or *sedimentum lateritium*), giving the sediment a characteristic pinkish hue. **Explanation of Incorrect Options:** * **A. Urochrome:** This is the primary pigment responsible for the typical yellow color of urine. It is an oxidation product of urobilinogen and its excretion rate is generally constant, meaning the intensity of the yellow color depends on the urine concentration (hydration status). * **B. Urobilin:** This is the oxidized form of urobilinogen. It contributes significantly to the yellow-amber pigment of urine. * **D. Urobilinogen:** While colorless itself, urobilinogen is a precursor that readily oxidizes into urobilin upon exposure to air and light, contributing to the overall yellow spectrum of voided urine. **High-Yield Clinical Pearls for NEET-PG:** * **Urobilinogen vs. Bilirubin:** In **hemolytic jaundice**, urine urobilinogen is increased (darker yellow), but bilirubin is absent (acholuric jaundice). In **obstructive jaundice**, bilirubin is present (tea-colored urine), but urobilinogen is absent. * **Alkaptonuria:** Urine turns black on standing due to the oxidation of homogentisic acid. * **Porphyria:** Urine may turn "port-wine" red upon exposure to light due to the formation of porphobilin. * **Dietary influence:** Consumption of beets can cause **beeturia** (pink/red urine), which must be differentiated from hematuria.

Q: A patient presents with unconjugated hyperbilirubinemia and elevated urobilinogen levels in urine. What is the most likely diagnosis?

Hemolytic Jaundice. The clinical presentation of **unconjugated hyperbilirubinemia** combined with **elevated urinary urobilinogen** is a classic hallmark of **Hemolytic Jaundice**. [1] **1. Why Hemolytic Jaundice is correct:** In hemolysis, there is an excessive breakdown of hemoglobin, leading to an overproduction of unconjugated bilirubin (UCB). The liver conjugates as much as possible and excretes it into the bile. In the intestine, this excess bilirubin is converted by bacteria into **stercobilinogen/urobilinogen**. A significant portion of this increased urobilinogen is reabsorbed into the portal circulation (enterohepatic circulation) and subsequently excreted by the kidneys, leading to high urinary urobilinogen levels. [1] Notably, urine bilirubin is absent (acholuric jaundice) because UCB is water-insoluble. **2. Why other options are incorrect:** * **Gilbert’s & Crigler-Najjar Syndromes:** These are genetic defects in the conjugation enzyme (UGT1A1). [1] While they cause unconjugated hyperbilirubinemia, the total amount of bilirubin reaching the gut is **decreased or normal**, meaning urobilinogen levels will not be elevated. * **Dubin-Johnson Syndrome:** This is a conjugated hyperbilirubinemia caused by a defect in bilirubin excretion into the bile canaliculi. [1] It would present with bilirubinuria (dark urine) rather than isolated urobilinogen elevation. **High-Yield NEET-PG Pearls:** * **Hemolysis:** ↑ UCB, ↑ Urinary Urobilinogen, **Absent** Urinary Bilirubin. [1] * **Obstructive Jaundice:** ↑ Conjugated Bilirubin, **Absent** Urinary Urobilinogen (bile doesn't reach the gut), **Present** Urinary Bilirubin. [2] * **Gilbert Syndrome:** Most common hereditary hyperbilirubinemia; jaundice is typically triggered by fasting or stress. [1] * **Dubin-Johnson:** Characterized by a **black liver** on biopsy due to melanin-like pigment.

Question 1

Which of the following statements is true regarding the binding of O2 to hemoglobin?

Accepted Answer

Is cooperative, meaning that after the first O2 binds, the other subunits are more readily oxygenated.

Answer

Results in a release of the heme from the interior to the exterior of the Beta subunits, leading to an increase in O2 affinity of the alpha subunits.

Answer

Causes a large shift of the surrounding secondary structures, which leads to decreased affinity of the deoxy subunits for CO2.

Answer

Occurs with equal affinity at all four subunits.

Question 2

Which of the following statements is NOT true regarding the buffering action of hemoglobin?

Accepted Answer

Hemoglobin's buffering action is primarily extracellular, not functional in the plasma.

Answer

Hemoglobin functions as an intracellular buffer.

Answer

Hemoglobin's buffering capacity is significantly attributed to its high histidine content.

Answer

Oxygenated hemoglobin acts as a stronger base compared to deoxygenated hemoglobin.

Question 3

Study the given oxygen-hemoglobin dissociation curve and comment on the finding shown, identifying which protein corresponds to curve A and which to curve B.

Accepted Answer

Curve B represents myoglobin, and curve A represents hemoglobin.

Answer

Curve A represents myoglobin, and curve B represents hemoglobin.

Answer

Both curves A and B represent myoglobin.

Answer

Both curves A and B represent hemoglobin.

Question 4

A 10-year-old boy presented with muscle weakness and fatigue with increased lead in the blood. Which enzyme production in the liver is increased?

Accepted Answer

ALA synthase

Answer

Heme oxygenase

Answer

Ferrochelatase

Answer

Porphobilinogen deaminase

Question 5

Hemoglobin electrophoresis in SC A (sickle cell anemia) shows the presence of which hemoglobin?

Accepted Answer

HbA

Answer

HbF

Answer

HbA2

Answer

None of the above

Question 6

A missense gene mutation in the beta-globin chain is characteristic of which condition?

Accepted Answer

Sickle cell anemia

Answer

Thalassemia

Answer

Hemoglobin Bart's (Hb Barts)

Answer

Hemoglobin H (HbH)

Question 7

An Rh-negative woman delivered an Rh-positive infant. Within a few days after birth, the infant developed jaundice, ascites, hepatomegaly, and edema. What is the likely substance deposited in the skin and sclera in jaundice?

Accepted Answer

Conjugated and unconjugated bilirubin

Answer

Biliverdin

Answer

Unconjugated bilirubin

Answer

Conjugated bilirubin

Question 8

Iron in heme is linked to globin through which amino acid residue?

Accepted Answer

Histidine

Answer

Alanine

Answer

Glycine

Answer

Cysteine

Question 9

Pale lemon yellow color of urine is due to all except?

Accepted Answer

Uroerythrin

Answer

urochrome

Answer

urobilin

Answer

Urobilinogen

Question 10

A patient presents with unconjugated hyperbilirubinemia and elevated urobilinogen levels in urine. What is the most likely diagnosis?

Accepted Answer

Hemolytic Jaundice

Answer

Crigler-Najjar syndrome

Answer

Gilbert's syndrome

Answer

Dubin-Johnson syndrome

Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?