USMLE Step 1 Question
TOPIC: Pulmonary Embryology
The following is a high-yield USMLE Step 1 exam question.
To get the most out of this practice question deep-dive, I recommend doing the following:
Go through the question using our test-taking strategy (Not sure how to use it? Read about it HERE)
Formulate your answer and find it in the answer choices
If you believe you’ve identified the correct answer, you can scroll to the bottom of the post to see if you’re right; if you are - congrats! Be sure to read through the deep-dive section to extract more knowledge from the question
If you aren’t 100% sure about the answer, start with the deep-dive section to get clear on the concept(s), then try to formulate an answer from there; once you believe you’ve found the answer, scroll to the bottom of the page (if you were wrong, re-read the deep-dive to figure out why).
QUESTION:
A 3-week-old infant is brought to the emergency department by concerned parents due to persistent difficulty in feeding, episodes of cyanosis, and noticeable breathing difficulties, particularly after feeding. The infant, born full-term with no complications reported during delivery, has exhibited poor weight gain since birth. Physical examination reveals increased respiratory effort with evident subcostal and intercostal retractions. Breath sounds are diminished on the left side of the chest. An urgent chest X-ray shows the presence of bowel loops within the left hemithorax and a mediastinal shift to the right. The patient’s condition has most likely resulted from an error during which of the following stages of lung development?
A. Embryonic stage
B. Pseudoglandular stage
C. Canalicular stage
D. Saccular stage
E. Alveolar stage
The Mental Model of answering this question:
Step 1 - State your primary impression based on the patient’s presentation
Step 2 - Make a diagnosis of the patient’s condition based on presentation
Step 3 - Formulate an answer that directly answers the question posed in the last line of the vignette.
DEEP-DIVE:
PRIMARY IMPRESSION: The clinical presentation of a 3-week-old male with feeding difficulties, cyanosis, respiratory distress, poor weight gain, and specific radiographic findings such as bowel loops in the thoracic cavity and mediastinal shift are characteristic of congenital diaphragmatic hernia (CDH).
DIAGNOSIS: Congenital diaphragmatic hernia
FORMULATION OF AN ANSWER TO THE LAST LINE: Based on a likely diagnosis of CDH, the most likely answer should reflect a 1st-trimester error in development.
Let’s take a detailed look at the answer choices:
A. Embryonic Stage
Timing: Weeks 3-8 of gestation.
Key Developments: Formation of the lung bud from the foregut, initial branching of the trachea and lung buds, and establishment of the basic lung structure.
Clinical Relevance: Errors during this stage can lead to major congenital anomalies like tracheoesophageal fistulae and congenital diaphragmatic hernia, as the separation of different organ systems occurs during this period.
B. Pseudoglandular Stage
Timing: Weeks 5-17 of gestation.
Key Developments: The airways branch further down to the terminal bronchioles, resembling a glandular structure. No alveoli or gas exchange regions are present yet.
Clinical Relevance: While the lung is structurally formed, it is not capable of gas exchange. Anomalies during this stage can affect the bronchial tree's architecture, potentially leading to bronchial atresia or pulmonary hypoplasia.
C. Canalicular Stage
Timing: Weeks 16-26 of gestation.
Key Developments: Development of respiratory bronchioles and beginning of alveolar ducts, increased vascularization, and the beginning capability for gas exchange.
Clinical Relevance: Disruptions in this stage can severely impact the lung's capacity for gas exchange at birth, leading to conditions like respiratory distress syndrome in premature infants.
D. Saccular Stage
Timing: Weeks 24 of gestation to birth.
Key Developments: Formation and enlargement of saccules at the end of the airways, increase in capillary networks, and thinning of the tissue between the air spaces and blood, preparing the lung for air breathing.
Clinical Relevance: Insufficient development during this stage can lead to inadequate surfactant production and reduced surface area for gas exchange, contributing to respiratory distress post-birth.
E. Alveolar Stage
Timing: Late gestation (around 36 weeks) through several years after birth.
Key Developments: Rapid increase in the number of alveoli and maturation of the alveoli for efficient gas exchange.
Clinical Relevance: Postnatal alveolar development is crucial for the ongoing adaptation to air breathing; disruption can impact long-term respiratory function and is linked to chronic lung diseases of infancy.
Considering the diagnosis and the timeframe within which a congenital diaphragmatic hernia is most likely to develop (~ 8th week), the patient’s findings are most likely associated with the ‘embryonic’ stage of development.
FINAL ANSWER: A) Embryonic stage
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