COMSAE - Comprehensive Osteopathic Medical Self-Assessment Examination Microbiology and Immunology Questions and Answers — Questions and Answers
Question 1: A 19-year-old male sustains a deep laceration from a piece of metal while working in a machine shop. Within hours, the area becomes red, swollen, and warm. Which of the following cells is the primary mediator of the initial, non-specific inflammatory response by migrating first to the site of injury?
- Macrophage
- Neutrophil (Correct answer)
- B-lymphocyte
- Natural Killer (NK) cell
Correct answer: Neutrophil
Neutrophils are the most abundant type of white blood cell and are the first responders of the innate immune system to sites of acute inflammation and tissue injury. Their primary roles include phagocytosis of pathogens and debris, and the release of antimicrobial factors to combat infection. Macrophages arrive later, B-lymphocytes are part of the adaptive immune response, and NK cells primarily target virally infected cells and tumor cells.
Question 2: A 25-year-old hiker presents with a pruritic, erythematous, and vesicular rash on his forearms that appeared 48 hours after clearing a trail. He recalls brushing against several plants. This clinical presentation is characteristic of which type of hypersensitivity reaction?
- Type I (Anaphylactic)
- Type II (Cytotoxic)
- Type IV (Delayed-type) (Correct answer)
- Type III (Immune complex)
Correct answer: Type IV (Delayed-type)
The rash described is classic for allergic contact dermatitis, such as from poison ivy. This is a Type IV, or delayed-type, hypersensitivity reaction. It is mediated by T-lymphocytes, not antibodies, and characteristically takes 24-72 hours to develop after re-exposure to the antigen (in this case, urushiol oil from the plant). Type I is immediate and IgE-mediated, Type II is antibody-mediated against cell surfaces, and Type III involves deposition of antigen-antibody complexes.
Question 3: A 3-year-old boy presents with his fourth episode of pneumonia in the past year, in addition to recurrent otitis media. The causative organisms have consistently been encapsulated bacteria like *Streptococcus pneumoniae*. Physical exam is notable for an absence of tonsillar tissue. Laboratory studies show markedly decreased levels of IgG, IgA, and IgM. Which of the following is the most likely diagnosis?
- Severe Combined Immunodeficiency (SCID)
- Chronic Granulomatous Disease (CGD)
- DiGeorge Syndrome
- X-linked (Bruton) Agammaglobulinemia (XLA) (Correct answer)
Correct answer: X-linked (Bruton) Agammaglobulinemia (XLA)
X-linked Agammaglobulinemia (XLA) is a primary immunodeficiency characterized by a failure of B-cell precursors to mature, leading to a profound deficiency of all immunoglobulin isotypes. This results in recurrent sinopulmonary infections with encapsulated bacteria, typically beginning after 6 months of age as maternal antibodies wane. The absence of B-cells also leads to underdeveloped lymphoid tissues, such as tonsils. SCID affects both B and T cells, CGD is a defect in phagocyte function, and DiGeorge syndrome is primarily a T-cell deficiency.
Question 4: Which of the following virulence factors of *Staphylococcus aureus* directly impairs opsonization and phagocytosis by binding to the Fc region of IgG?
- Protein A (Correct answer)
- Coagulase
- Hyaluronidase
- Exfoliative toxin
Correct answer: Protein A
Protein A is a surface protein on *S. aureus* that binds to the Fc portion of IgG antibodies. This incorrect orientation prevents the antibody from being recognized by phagocyte Fc receptors, thereby inhibiting opsonization and phagocytosis, which is a key immune evasion mechanism. Coagulase promotes fibrin clotting, hyaluronidase breaks down connective tissue, and exfoliative toxin causes skin blistering.
Question 5: A physician is counseling the parents of a 5-month-old infant diagnosed with Severe Combined Immunodeficiency (SCID). Which of the following routine childhood immunizations is strictly contraindicated in this patient?
- Diphtheria-Tetanus-acellular Pertussis (DTaP)
- Inactivated Poliovirus (IPV)
- Measles-Mumps-Rubella (MMR) (Correct answer)
- Hepatitis B (HepB)
Correct answer: Measles-Mumps-Rubella (MMR)
The Measles-Mumps-Rubella (MMR) vaccine is a live-attenuated vaccine. Live vaccines are contraindicated in individuals with severe cellular immunodeficiency, such as SCID, because the weakened pathogen can replicate unchecked and cause a life-threatening infection. DTaP, IPV, and Hepatitis B are all inactivated or subunit vaccines and are safe to administer to immunocompromised patients, though the immune response may be suboptimal.
Question 6: The life cycle of the Human Immunodeficiency Virus (HIV) requires the synthesis of a DNA copy from its RNA genome, which is then integrated into the host cell's chromosome. This process is mediated by which of the following viral enzymes?
- Protease
- Integrase
- RNA polymerase
- Reverse transcriptase (Correct answer)
Correct answer: Reverse transcriptase
Reverse transcriptase is the key enzyme used by retroviruses like HIV to convert their single-stranded RNA genome into double-stranded DNA (a process called reverse transcription). This DNA provirus can then be integrated into the host genome by the viral enzyme integrase. Protease is involved later in the viral life cycle, cleaving viral polyproteins into their functional forms. Host cell RNA polymerase is used to transcribe the integrated viral DNA.
COMSAE - Comprehensive Osteopathic Medical Self-Assessment Examination Microbiology and Immunology Questions and Answers
A 19-year-old male sustains a deep laceration from a piece of metal while working in a machine shop.
Within hours, the area becomes red, swollen, and warm.
Which of the following cells is the primary mediator of the initial, non-specific inflammatory response by migrating first to the site of injury?