USMLE PRACTICE QUESTION #57
TOPIC: Immunology & Infectious Disease
Today’s question will test your understanding of Immunology and infectious disease.
I’m Paul Ciurysek, MD, and this is The USMLE Times: An independent, subscriber-supported newsletter designed to help you survive medical school, pass your USMLE exams, and match into your dream residency program.
A 29-year-old immunocompetent woman presents to her primary care physician with complaints of intermittent vaginal itching and discomfort over the past month. She denies any discharge, fever, or pain. Her vital signs are as follows: Blood pressure 120/75 mmHg, pulse 68 beats/minute, respiratory rate 14 breaths/minute, and temperature 98.6°F (37°C). A pelvic examination reveals a small cauliflower-like lesion on the cervix. Which of the following is the most relevant factor in preventing initial infection?
A. Impaired activation of natural killer (NK) cells
B. Inadequate phagocytosis by macrophages
C. Inhibition of the complement system
D. Decreased secretion of antimicrobial peptides by epithelial cells
E. Suppressed activation of cytotoxic T cells
F. Insufficient secretion of IgA in cervical mucus
G. Reduced production of IFN-γ by Th1 cells
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The mental model used to answer this question comes from our detailed test-taking skills masterclass. Here’s how to think through this question:
Step 1. Read the last line to determine what the question is asking: What’s the cause of increased physiologic dead space in the patient?
Step 2: Is this a first-, second-, or third-order question?
Answer: 3rd order: 1st. Recognize the type of infection 2nd. Understand the mechanisms of infection and 3rd. Determine which mechanism played the most important initial role in infection.
Step 3. Read the vignette to gather information.
Step 4. Consider the question and ask yourself the most likely answer.
Step 5. Look at the answer choices and select the option most closely resembling your thoughts from “Step 4” above.
GENERAL ANALYSIS
The question involves a 29-year-old immunocompetent woman presenting with a cauliflower-like lesion on her cervix, which is indicative of a human papillomavirus (HPV) infection leading to cervical dysplasia or cancer. The task is to identify the factor that likely contributed to her inability to prevent this infection despite being immunocompetent.
ANSWER CHOICES:
ANSWER CHOICE A: Impaired activation of natural killer (NK) cells
Explanation: NK cells play a role in controlling viral infections and tumor surveillance. While NK cells are important, they are not the primary defense against HPV infections.
ANSWER CHOICE B: Inadequate phagocytosis by macrophages
Explanation: Macrophages are involved in phagocytosis and antigen presentation. HPV primarily affects epithelial cells, and macrophage activity is not the primary defense mechanism against this virus.
ANSWER CHOICE C: Inhibition of the complement system
Explanation: The complement system enhances the ability of antibodies and phagocytic cells to clear pathogens. It is not directly involved in the defense against HPV infections.
ANSWER CHOICE D: Decreased secretion of antimicrobial peptides by epithelial cells
Explanation: Antimicrobial peptides provide a first line of defense against pathogens at mucosal surfaces. While important, they are not specifically linked to preventing HPV infections.
ANSWER CHOICE E: Suppressed activation of cytotoxic T cells
Explanation: Cytotoxic T cells are crucial for targeting and destroying virus-infected cells. HPV evades the immune system by downregulating MHC class I molecules, which impairs cytotoxic T-cell recognition and response. This mechanism allows HPV to persist in the host.
ANSWER CHOICE F: Insufficient secretion of IgA in cervical mucus
Explanation: IgA is important for mucosal immunity and provides a barrier against pathogens at mucosal surfaces. Adequate IgA levels can help prevent viral entry and establishment at these sites.
ANSWER CHOICE G: Reduced production of IFN-γ by Th1 cells
Explanation: IFN-γ is important for activating macrophages and enhancing antigen presentation. While IFN-γ plays a role in immune responses, it is not the primary issue associated with HPV infection.
FINAL VERDICT…
While both immune mechanisms are essential, HPV's ability to evade the immune system is more closely associated with its effect on cellular immunity, particularly through the evasion of cytotoxic T-cell responses. However, IgA plays a significant role in preventing initial infection by neutralizing viruses at mucosal surfaces. In this context, insufficient IgA could indeed contribute to the establishment of HPV infection by failing to prevent the virus from accessing epithelial cells.
Given this reassessment, insufficient secretion of IgA could be a more relevant factor in preventing initial infection or reinfection at mucosal surfaces, especially considering that IgA provides a first line of defense against pathogens entering through mucosal routes.
CORRECT ANSWER: F) Insufficient secretion of IgA in cervical mucus
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Thanks for checking out Q56!
See you next time 👋