Mechanism of phagocytosis MCQs With Answer

Mechanism of phagocytosis MCQs With Answer

This quiz is designed for M.Pharm students to deepen understanding of the cellular and molecular mechanisms of phagocytosis. Questions cover receptor recognition, signaling cascades, cytoskeletal dynamics, phagosome maturation, intracellular killing mechanisms (respiratory burst, reactive nitrogen species, myeloperoxidase activity), and defects leading to disease. Emphasis is placed on pharmacologically relevant targets, opsonins, complement and Fc receptor interactions, Rab proteins, v‑ATPase–dependent acidification, LC3-associated phagocytosis, and clinical correlations such as chronic granulomatous disease. Use these MCQs to test and reinforce advanced conceptual knowledge required for immunotechnology and therapeutic intervention design.

Q1. Which of the following receptors primarily mediates opsonin-dependent phagocytosis of IgG-coated particles?

• Complement receptor 3 (CR3)
• Scavenger receptor A
• Fc gamma receptor I (FcγRI/CD64)
• Toll-like receptor 4 (TLR4)

Correct Answer: Fc gamma receptor I (FcγRI/CD64)

Q2. Opsonization by complement most commonly involves which complement fragment binding directly to a target to enhance phagocytosis?

• C5a
• C3b
• C1q
• C9

Correct Answer: C3b

Q3. Activation of NADPH oxidase during the respiratory burst leads to generation of which primary reactive species within the phagosome?

• Hydrogen peroxide (H2O2)
• Superoxide anion (O2•−)
• Hypochlorous acid (HOCl)
• Peroxynitrite (ONOO−)

Correct Answer: Superoxide anion (O2•−)

Q4. Which enzyme is responsible for converting H2O2 and chloride into the highly microbicidal hypochlorous acid inside neutrophil phagolysosomes?

• Inducible nitric oxide synthase (iNOS)
• Superoxide dismutase (SOD)
• Myeloperoxidase (MPO)
• Cathepsin D

Correct Answer: Myeloperoxidase (MPO)

Q5. Which Rab GTPase is associated with early phagosomes and is important for early endosomal fusion events?

• Rab7
• Rab5
• Rab11
• Rab1

Correct Answer: Rab5

Q6. Acidification of the phagosome is primarily achieved by which molecular complex?

• Vacuolar-type H+-ATPase (v‑ATPase)
• Na+/K+ ATPase
• ATP synthase
• Proton channel Hv1

Correct Answer: Vacuolar-type H+-ATPase (v‑ATPase)

Q7. LC3-associated phagocytosis (LAP) differs from canonical autophagy in that LAP directly recruits LC3 to phagosomal membranes in response to which stimulus?

• Starvation-induced nutrient deprivation
• Recognition of single-membrane phagosomes containing pathogens or apoptotic cells
• ER stress
• Mitophagy of damaged mitochondria

Correct Answer: Recognition of single-membrane phagosomes containing pathogens or apoptotic cells

Q8. Defect in which component of the NADPH oxidase complex is most commonly associated with X-linked chronic granulomatous disease (CGD)?

• p47phox (NCF1)
• gp91phox (CYBB)
• p22phox (CYBA)
• p67phox (NCF2)

Correct Answer: gp91phox (CYBB)

Q9. Phagosome–lysosome fusion requires SNARE proteins and which small GTPase that controls late phagosome maturation?

• Rab5
• Rab7
• RhoA
• Rac1

Correct Answer: Rab7

Q10. Which signaling molecule is critical for actin polymerization and formation of the phagocytic cup downstream of Fcγ receptor engagement?

• PI3K generating PIP2
• PI3K generating PIP3 and activation of Rac/Cdc42
• cAMP-dependent protein kinase (PKA)
• p53 activation

Correct Answer: PI3K generating PIP3 and activation of Rac/Cdc42

Q11. Which of the following best describes the role of opsonins in phagocytosis?

• Directly neutralize toxins released by pathogens
• Label particles to enhance recognition by phagocyte receptors
• Destabilize microbial membranes via pore formation
• Prevent antigen presentation by dendritic cells

Correct Answer: Label particles to enhance recognition by phagocyte receptors

Q12. Which receptor recognizes iC3b and facilitates complement-mediated phagocytosis by neutrophils and macrophages?

• CR1 (CD35)
• CR2 (CD21)
• CR3 (Mac-1, CD11b/CD18)
• CR4 (CD11c/CD18)

Correct Answer: CR3 (Mac-1, CD11b/CD18)

Q13. Nitric oxide (NO) produced by inducible nitric oxide synthase in macrophages contributes to intracellular killing mainly by:

• Alkalinizing the phagosomal lumen
• Directly oxidizing proteins and forming reactive nitrogen intermediates
• Inhibiting lysosomal enzyme activity
• Sequestering iron required for microbial growth

Correct Answer: Directly oxidizing proteins and forming reactive nitrogen intermediates

Q14. Which of the following events is a hallmark of frustrated phagocytosis?

• Efficient internalization and formation of phagolysosome
• Failure to engulf large target with extracellular release of lytic enzymes
• Lysosomal biogenesis and antigen cross-presentation
• Activation of mitochondrial apoptosis in the phagocyte

Correct Answer: Failure to engulf large target with extracellular release of lytic enzymes

Q15. The proton channel Hv1 contributes to sustained NADPH oxidase activity by:

• Importing chloride into the phagosome
• Exporting protons to balance charge and prevent excessive cytosolic acidification
• Generating ATP for NADPH oxidase assembly
• Directly reducing superoxide to H2O2

Correct Answer: Exporting protons to balance charge and prevent excessive cytosolic acidification

Q16. Which lysosomal membrane protein is commonly used as a marker of late phagolysosomes?

• LAMP-1
• Clathrin heavy chain
• Calnexin
• Beclin-1

Correct Answer: LAMP-1

Q17. Toll-like receptor engagement during phagocytosis modulates phagosomal function primarily by:

• Blocking Rab GTPase recruitment to phagosomes
• Altering phagosomal signaling to enhance antigen processing and cytokine production
• Inhibiting NADPH oxidase assembly
• Preventing v‑ATPase recruitment

Correct Answer: Altering phagosomal signaling to enhance antigen processing and cytokine production

Q18. Which cytoskeletal small GTPase promotes membrane protrusions and actin remodeling required for engulfment in phagocytosis?

• RhoA
• Rac1
• Ran
• Rab5

Correct Answer: Rac1

Q19. Antigen cross-presentation of phagocytosed material on MHC class I molecules occurs via which key mechanism in dendritic cells?

• Direct loading of intact antigens onto surface MHC I without processing
• Transfer of phagosomal peptides into the cytosol for proteasomal processing and TAP-dependent loading
• Exclusive use of lysosomal proteases to generate peptides for MHC I
• Sequestration of antigens in autophagosomes that fuse with the ER

Correct Answer: Transfer of phagosomal peptides into the cytosol for proteasomal processing and TAP-dependent loading

Q20. Pharmacological inhibition of PI3K during phagocytosis is most likely to impair which step?

• Fusion of lysosomes with mitochondria
• Actin remodeling and phagocytic cup closure
• Translation of cytokine mRNA
• Release of histamine from mast cells

Correct Answer: Actin remodeling and phagocytic cup closure

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