Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 211: A 25-year-old patient presents with cyanosis and is diagnosed with methemoglobinemia. What is the underlying biochemical mechanism responsible for this condition?

A. Oxidation of Fe2+ to Fe3+ in hemoglobin (Correct Answer)
B. Increased synthesis of hemoglobin
C. Reduction of Fe3+ to Fe2+ in hemoglobin
D. Decreased oxygen affinity of hemoglobin

Explanation: ***Oxidation of Fe2+ to Fe3+ in hemoglobin*** - **Methemoglobinemia** occurs when the **ferrous iron (Fe2+)** in the heme group of hemoglobin is oxidized to its **ferric state (Fe3+)**. - **Ferric iron** cannot bind oxygen, leading to a functional anemia and **cyanosis**. *Increased synthesis of hemoglobin* - This condition would typically lead to **polycythemia**, an increase in red blood cell count and hemoglobin, not cyanosis due to impaired oxygen binding. - Increased hemoglobin synthesis would generally enhance oxygen-carrying capacity, which is the opposite of what is observed in methemoglobinemia. *Reduction of Fe3+ to Fe2+ in hemoglobin* - This process is the normal mechanism by which **methemoglobin reductase enzymes** convert methemoglobin back to functional hemoglobin. - An effective reduction of **Fe3+ to Fe2+** would actually prevent or resolve methemoglobinemia, rather than cause it. *Decreased oxygen affinity of hemoglobin* - While methemoglobin does not bind oxygen, a condition of generally **decreased oxygen affinity** (e.g., due to a right shift in the oxygen-hemoglobin dissociation curve) can be caused by factors like **2,3-BPG** or acidosis, but it does not directly explain the formation of methemoglobin. - Decreased oxygen affinity would lead to oxygen release in tissues but not necessarily to the characteristic slate-gray cyanosis seen with methemoglobin, which is due to the non-functional ferric iron.

Question 212: A mutation leads to a significant decrease in the cooperativity of hemoglobin for oxygen binding. Which allosteric regulator's interaction is most likely disrupted?

A. 2,3-Bisphosphoglycerate (Correct Answer)
B. Carbon dioxide
C. Hydrogen ions
D. Carbon monoxide

Explanation: ***2,3-Bisphosphoglycerate*** - **2,3-BPG** binds to an **allosteric site** (pocket formed by the beta subunits) of deoxyhemoglobin, stabilizing the **tense (T) state** and reducing its affinity for oxygen. - A mutation disrupting 2,3-BPG binding would impair the normal allosteric regulation of the T↔R state transition, **decreasing cooperativity** by preventing proper stabilization of the low-affinity T-state. - This is the primary physiological allosteric regulator whose interaction, when disrupted, would specifically decrease cooperativity. *Carbon dioxide* - **Carbon dioxide** primarily affects hemoglobin's oxygen affinity by forming **carbamates** with the N-termini of globin chains and by influencing pH (Bohr effect). - While CO2 is an allosteric regulator, its direct impact on the cooperativity mechanism is less pronounced compared to 2,3-BPG's specific role in modulating the T↔R state transition. *Hydrogen ions* - **Hydrogen ions** (protons) decrease hemoglobin's oxygen affinity by binding to specific amino acid residues, stabilizing the **tense (T) state** (the **Bohr effect**). - Although H+ is an allosteric regulator, a mutation specifically causing decreased cooperativity points more directly to 2,3-BPG, which has the most significant role in modulating the sigmoidal binding curve. *Carbon monoxide* - **Carbon monoxide** binds **competitively to the heme iron active site** (NOT an allosteric regulatory site) with an affinity 200-250 times higher than oxygen. - While CO binding does abolish cooperativity by locking hemoglobin in a high-affinity state, this occurs through **direct competitive inhibition** at the oxygen binding site, not through disruption of an allosteric regulator's interaction. - The question specifically asks about allosteric regulator disruption, making this incorrect despite its effect on cooperativity.

Question 213: A patient with jaundice has elevated unconjugated bilirubin levels. Which enzyme is most likely deficient?

A. Glucuronyl transferase (Correct Answer)
B. Heme oxygenase
C. Biliverdin reductase
D. Heme synthase

Explanation: ***Glucuronyl transferase*** - This enzyme is responsible for **conjugating bilirubin** with glucuronic acid, converting **unconjugated (indirect) bilirubin** into **conjugated (direct) bilirubin**. - A deficiency in glucuronyl transferase would lead to an accumulation of **unconjugated bilirubin**, causing **jaundice**. *Heme oxygenase* - This enzyme breaks down **heme** into **biliverdin**, which is then converted to bilirubin. - A deficiency would lead to *decreased* bilirubin production, not an *increase* in unconjugated bilirubin. *Biliverdin reductase* - This enzyme converts **biliverdin** into **unconjugated bilirubin**. - A deficiency would impair the production of bilirubin from biliverdin, thus *decreasing* unconjugated bilirubin levels. *Heme synthase* - This enzyme is involved in the final step of **heme synthesis**, inserting iron into protoporphyrin. - A deficiency would primarily affect **heme production** and could lead to **porphyrias** or **anemia**, not elevated unconjugated bilirubin.

Question 214: Which pigment is responsible for the greenish-black color of neonatal stool?

A. Biliverdin (Correct Answer)
B. Urochrome
C. Stercobilin
D. Bilirubin (yellow pigment)

Explanation: ***Biliverdin*** - **Biliverdin** is a green pigment formed from the breakdown of heme before it is converted to bilirubin, and it is responsible for the greenish-black color of **meconium**. - The presence of this pigment in the stool indicates the passage of **meconium**, the first stool of a newborn. *Urochrome* - **Urochrome** is responsible for the yellow color of **urine**, not stool. - It is a pigment derived from **bilirubin** that is excreted by the kidneys. *Stercobilin* - **Stercobilin** is responsible for the characteristic **brown color of adult feces**. - It is formed when **bilirubin** is metabolized by bacteria in the intestine. *Bilirubin (yellow pigment)* - **Bilirubin** is typically a **yellow-orange pigment**, not greenish-black. - While bilirubin is the precursor to stercobilin, its yellow form is more associated with **jaundice** when present in high concentrations.

Question 215: What is the approximate daily iron requirement for a healthy Indian male?

A. 17 mg (Correct Answer)
B. 10 mg
C. 35 mg
D. 5 mg

Explanation: ***17 mg*** - The **Indian Council of Medical Research (ICMR)** recommends approximately **17 mg/day** of iron for adult Indian males to account for the predominantly **vegetarian diet** in India. - Indian diets are rich in **phytates, tannins, and dietary fiber** which significantly reduce iron bioavailability (non-heme iron absorption is only 5-10% compared to 15-25% for heme iron). - This higher recommendation ensures adequate iron absorption to maintain **hemoglobin synthesis** and prevent iron deficiency despite lower bioavailability. *10 mg* - This is the recommendation for populations with predominantly **non-vegetarian diets** (like Western countries) where heme iron from meat has higher bioavailability. - For the **Indian context**, this amount would be **insufficient** given the lower bioavailability of iron from plant-based sources. - While adequate for Western populations, it does not account for India-specific dietary patterns. *35 mg* - This is a **therapeutic dose** used for treating iron deficiency anemia or during pregnancy, not a routine daily requirement. - Such high doses are prescribed under medical supervision for specific clinical conditions and could lead to **gastrointestinal side effects** or iron overload with chronic use. *5 mg* - This amount is **grossly insufficient** for any adult male population and would lead to **negative iron balance** and eventual iron deficiency. - Daily iron losses through desquamation, sweat, and GI tract are approximately 1 mg, requiring adequate dietary intake to maintain iron homeostasis.

Question 216: Which of the following statements about coproporphyrin I and coproporphyrin III is correct?

A. In Dubin Johnson Syndrome, Coproporphyrin I in urine is 80% of the total coproporphyrin (Correct Answer)
B. Coproporphyrin I is not normally excreted in urine.
C. In Dubin Johnson Syndrome, total coproporphyrin levels are elevated.
D. Coproporphyrin III is primarily excreted in urine.

Explanation: ***Correct: In Dubin Johnson Syndrome, Coproporphyrin I in urine is 80% of the total coproporphyrin*** - **Dubin-Johnson syndrome** is an inherited disorder characterized by a defect in the **MRP2 transporter** (multidrug resistance protein 2), which impairs hepatocellular excretion of conjugated bilirubin and other organic anions, including **coproporphyrin III**. - This defect leads to the preferential excretion of **coproporphyrin I** into the urine, causing a **reversal of the normal ratio**. - In Dubin-Johnson syndrome, **coproporphyrin I comprises >80%** of total urinary coproporphyrin (vs. normal 20-25%). - This is a **key diagnostic feature** of the condition. *Incorrect: Coproporphyrin I is not normally excreted in urine.* - **Both coproporphyrin I and III** are normally excreted in urine, but in different proportions. - The healthy urinary excretion ratio of **coproporphyrin III to I is approximately 3-4:1** (meaning coproporphyrin I is about 20-25% and III is about 75-80% of total urinary coproporphyrin). - Coproporphyrin I is definitely present in normal urine, just in smaller amounts. *Incorrect: In Dubin Johnson Syndrome, total coproporphyrin levels are elevated.* - In Dubin-Johnson syndrome, the **total urinary coproporphyrin levels remain normal**. - What changes is the **ratio** of coproporphyrin I to III, not the total amount. - The key diagnostic feature is the **predominance of coproporphyrin I** (>80%), not an elevation in total coproporphyrin. *Incorrect: Coproporphyrin III is primarily excreted in urine.* - While **coproporphyrin III** is the predominant isomer normally found in urine (about 75-80% of total urinary coproporphyrin), its **primary excretion route is biliary**, not urinary. - A significant portion of coproporphyrin III is excreted via **bile into feces**, which is the major excretion pathway. - In contrast, **coproporphyrin I** has more balanced excretion between bile and urine in healthy individuals.

Question 217: What is the maximum daily degradation of hemoglobin in normal adults?

A. 8 gm
B. 6 gm (Correct Answer)
C. 2 gm
D. 4 gm

Explanation: ***6 gm*** - Approximately **6 grams of hemoglobin** are degraded daily in normal adults, primarily due to the breakdown of senescent red blood cells. - With an RBC lifespan of **120 days** and total body hemoglobin of approximately 750 grams, roughly **1% of RBCs are destroyed daily**. - This degradation process releases iron for reuse and converts the heme portion into **bilirubin**, which is then excreted. *2 gm* - This value is significantly **lower** than the actual amount of hemoglobin degraded daily. - Such a low degradation rate would imply a much longer red blood cell lifespan or a slower red blood cell turnover. *4 gm* - While closer, 4 grams is still an **underestimation** of the typical daily hemoglobin turnover in healthy adults. - This amount would not fully account for the normal destruction rate of roughly 1% of red blood cells per day. *8 gm* - This value is at the **upper limit** of normal daily hemoglobin degradation and is generally higher than the average in healthy individuals. - A persistent degradation of 8 grams or more might suggest an increased rate of **hemolysis** or other conditions leading to accelerated red blood cell destruction.

Question 218: What is the normal range of daily fecal urobilinogen excretion in healthy adults?

A. 40-280 mg (Correct Answer)
B. 100-200 mg
C. 20-40 mg
D. 50-300 mg

Explanation: ***40-280 mg*** - This range represents the **standard reference range** for daily fecal urobilinogen excretion in healthy adults, as cited in **Harper's Biochemistry** and **Vasudevan Biochemistry**. - **Urobilinogen** is a breakdown product of **bilirubin** formed by intestinal bacteria, with most being oxidized to **stercobilin** (brown pigment) in feces. - This is the **most commonly accepted range** in Indian medical textbooks and examination references. *20-40 mg* - This range is **significantly too low** for normal daily fecal urobilinogen excretion. - Values this low might suggest **decreased bile production**, **cholestasis**, **impaired gut flora activity**, or **antibiotic use** affecting intestinal bacteria. *100-200 mg* - While this falls within the normal range, it is **too narrow** and does not encompass the full variability seen in healthy individuals. - It excludes valid normal values below 100 mg and above 200 mg, making it an incomplete representation of the normal range. *50-300 mg* - This is an **alternative range** cited in some international references and is close to being correct. - However, **40-280 mg** is the **more precise and standard range** taught in Indian medical curricula and is preferred in competitive exam contexts (NEET PG, INI-CET). - The difference reflects slight variations in laboratory methods and population studies.

Question 219: Heme is synthesized from ?

A. Lysine + succinyl CoA
B. Glycine + succinyl CoA (Correct Answer)
C. Arginine + Malonyl CoA
D. Glycine + Malonyl CoA

Explanation: ***Glycine + succinyl CoA*** - Heme biosynthesis begins with the condensation of **glycine** and **succinyl CoA** to form **δ-aminolevulinic acid (ALA)**, catalyzed by **ALA synthase (ALAS)**. - This initial reaction is the **rate-limiting step** in heme synthesis and determines the overall production of heme. - This reaction occurs in the **mitochondria** and requires **pyridoxal phosphate (vitamin B6)** as a cofactor. *Lysine + succinyl CoA* - While succinyl CoA is a precursor, **lysine** is an essential amino acid involved in protein synthesis and other metabolic pathways, but not directly in heme synthesis. - The primary amino acid precursor for heme is **glycine**, not lysine. *Arginine + Malonyl CoA* - **Arginine** is involved in the urea cycle and nitric oxide synthesis, not heme synthesis. - **Malonyl CoA** is a key intermediate in **fatty acid synthesis**, not heme synthesis. *Glycine + Malonyl CoA* - **Glycine** is indeed a precursor for heme synthesis. - However, **malonyl CoA** is not involved in heme synthesis; rather, **succinyl CoA** is the correct coenzyme A derivative that condenses with glycine.

Question 220: In the context of iron metabolism, what does hepcidin inhibit?

A. Iron export from enterocytes (Correct Answer)
B. Absorption of cobalamine
C. Folic acid synthesis
D. Respiratory oxidase

Explanation: ***Iron export from enterocytes*** - **Hepcidin** is a key regulator of systemic iron homeostasis, primarily functioning by **inhibiting iron export** from enterocytes (duodenal mucosal cells). - It does this by binding to and inducing the degradation of **ferroportin**, the sole known iron exporter, thus reducing iron absorption into the bloodstream. *Absorption of cobalamine* - The absorption of **cobalamine (vitamin B12)** is primarily dependent on **intrinsic factor** secreted by gastric parietal cells, not hepcidin. - While iron and B12 deficiencies can coexist, hepcidin does not directly modulate B12 absorption. *Folic acid synthesis* - **Folic acid** (vitamin B9) is an essential nutrient that humans cannot synthesize; it must be obtained from the diet. - Hepcidin has no role in the synthesis or absorption of folic acid. *Respiratory oxidase* - **Respiratory oxidases** are enzymes involved in cellular respiration, particularly within the electron transport chain in mitochondria. - Hepcidin's action on iron metabolism is distinct from the function of these enzymes.

Practice by Chapter

Hemoglobin Structure and Function

Practice Questions

Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve

Practice Questions

Hemoglobin Variants and Hemoglobinopathies

Practice Questions

Thalassemias

Practice Questions

Methemoglobin and Abnormal Hemoglobins

Practice Questions

Hemoglobin Synthesis

Practice Questions

Heme Synthesis and Porphyrias

Practice Questions

Iron Absorption and Transport

Practice Questions

Iron Storage and Recycling

Practice Questions

Disorders of Iron Metabolism

Practice Questions

Anemia: Biochemical Aspects

Practice Questions

Biochemistry of Hemostasis

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free