USMLE Step 1 Question
TOPIC: 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 (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 4-day-old male is brought to the pediatrician by his parents due to difficulty breathing while feeding. The parents report that their son seems to breathe well through his mouth but struggles when attempting to breathe through his nose. They also notice that he becomes cyanotic while breastfeeding. On physical examination, the infant's vital signs are as follows: Heart rate 145/minute, respiratory rate 52/minute, temperature 98.6°F (37.0°C), and oxygen saturation 95% on room air. A nasogastric tube is inserted to assess the nasal airway, but the tube cannot pass through the right nostril. Which of the following embryological defects is most likely associated with the development of this condition?
A. Abnormal development of the first pharyngeal arch
B. Abnormal development of the third pharyngeal arch
C. Failure of the maxillary and medial nasal prominences to fuse
D. Failure of the lateral nasal prominences to fuse
E. Incomplete canalization of the nasal and oral cavities
F. Incomplete canalization of the nasal and nasopharyngeal cavities
DEEP-DIVE:
This question presents a clinical scenario involving a newborn with difficulty breathing through one nostril, particularly noticeable during feeding, leading to cyanosis. The inability to pass a nasogastric tube through the right nostril suggests a blockage or congenital anomaly in the nasal passage. Let's dissect each option to understand its implications and identify the correct answer.
A) Abnormal development of the first pharyngeal arch
Embryology: The first pharyngeal arch is crucial in forming the maxilla, mandible, malleus, incus, and muscles of mastication.
Potential Pathologies: Treacher Collins syndrome and Pierre Robin sequence.
Treacher-Collins Syndrome is primarily characterized by the underdevelopment of facial bones, leading to facial abnormalities.
Pierre Robin Sequence includes a set of features such as micrognathia, glossoptosis (tongue placed further back than usual), and airway obstruction.
These conditions involve genetic factors and developmental disruptions beyond just the first pharyngeal arch.
B) Abnormal development of the third pharyngeal arch
Embryology: The third pharyngeal arch contributes mainly to the formation of the hyoid bone's lower part and the stylopharyngeus muscle.
Potential Pathologies: Abnormalities in the third arch can lead to issues with swallowing and speech but are less commonly associated with well-defined syndromes. However, they could contribute to anomalies in the hyoid bone or anomalies in the development of the pharynx.
C) Failure of the maxillary and medial nasal prominences to fuse
Embryology: The fusion of the maxillary prominence with the medial nasal prominence is essential for the formation of the upper lip and primary palate.
Potential Pathologies: Failure of these structures to fuse can result in cleft lip and/or cleft palate, which can significantly affect feeding and speech and have associated dental issues.
D) Failure of the lateral nasal prominences to fuse
Embryology: The lateral nasal prominences play a role in forming the sides of the nose.
Potential Pathologies: Improper fusion could lead to broad or bifid nose deformities; however, these conditions are more cosmetic and structural than obstructive.
E) Incomplete canalization of the nasal and oral cavities
Embryology: Canalization is the process that forms the nasal and oral cavities by creating hollow spaces in the solid tissue.
Potential Pathologies: Incomplete canalization here can lead to choanal atresia, where the back of the nasal passage is blocked, affecting breathing. It can also contribute to other conditions like nasolacrimal duct obstruction, which can cause persistent tearing and infection.
F) Incomplete canalization of the nasal and nasopharyngeal cavities
Embryology: This process is responsible for creating a clear passageway from the nasal cavity to the nasopharynx.
Potential Pathologies: Incomplete canalization can result in choanal atresia, specifically posterior choanal atresia, which is a congenital blockage of the nasal passage by bone or tissue. This condition can lead to significant breathing difficulties, especially in newborns who are obligate nasal breathers.
THE VERDICT: Given the clinical scenario of a newborn with difficulty breathing through one nostril and an inability to pass a nasogastric tube through the same nostril, the most likely cause is the incomplete canalization of the nasal and nasopharyngeal cavities, leading to a condition such as choanal atresia. This condition affects the nasal passages and would directly lead to the symptoms described.
FINAL ANSWER: F. Incomplete canalization of the nasal and nasopharyngeal cavities
