USMLE PRACTICE QUESTION #102
TOPIC: Embryology.
Question #102 will test your understanding of a high-yield USMLE Step 1 embryology concept.
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A 27-year-old female G1 P1 delivers a male at 39 weeks gestation with a birth weight of 6 lb 10-oz and APGAR scores of 9 and 9 at 1 and 5 minutes, respectively. Upon physical examination, the neonate exhibits a fissure in the upper lip extending to the right nostril's base. Which of the following is the most likely cause of this finding?
A) Failed formation of the secondary palate
B) Failed fusion of the lateral palatine processes
C) Failed fusion of the medial palatine processes
D) Failed fusion of the maxillary and medial nasal prominences
E) Failed fusion of the palatine shelves with the primary palate
F) Failed fusion of the palatine shelves with the nasal septum
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 (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following is the most likely cause of this finding?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Determine what’s abnormal; 2nd: Determine the underlying cause of the abnormality.
Step 3: Read the vignette carefully and ask yourself: “Based on the patient’s presentation, I believe the most likely underlying error is _________”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This vignette presents a 27-year-old female who delivers a male infant at 39 weeks' gestation. The newborn has a fissure in the upper lip extending to the base of the right nostril—indicative of a unilateral cleft lip. The question asks for the most likely embryological cause of this condition.
ANSWER CHOICES:
CHOICE A: Failed formation of the secondary palate
Explanation: The secondary palate forms from the fusion of the lateral palatine processes and contributes to the formation of the hard and soft palate. This option refers to issues with palate formation, not the lip. A cleft palate results from failure in palatal fusion, not from failed formation of the secondary palate. This patient has a cleft lip, not a cleft palate. Therefore, this answer is incorrect.
CHOICE B: Failed fusion of the lateral palatine processes
Explanation: The lateral palatine processes fuse to form part of the secondary palate. This mechanism is related to cleft palate formation, not cleft lip. Since this patient has a cleft lip (not a cleft palate), this answer is incorrect.
CHOICE C: Failed fusion of the medial palatine processes
Explanation: The medial palatine processes contribute to forming part of the primary and secondary palate. Like option B, this answer refers to mechanisms involved in cleft palate formation rather than cleft lip. Therefore, this answer is incorrect.
CHOICE D: Failed fusion of the maxillary and medial nasal prominences
Explanation: During normal development, around week 7, the maxillary prominences fuse with the medial nasal prominences to form the upper lip. Failure of this fusion results in a cleft lip. The failure of fusion between these two structures results in an opening or fissure in the upper lip, as seen in this patient.
CHOICE E: Failed fusion of the palatine shelves with the primary palate
Explanation: The palatine shelves fuse with each other and with the primary palate to form part of the roof of the mouth. This mechanism is related to cleft palate formation, not cleft lip. Therefore, this answer is incorrect.
CHOICE F: Failed fusion of the palatine shelves with the nasal septum
Explanation: The palatine shelves must fuse with each other and with the nasal septum during development to form part of the hard and soft palate. Like options A, B, C, and E, this mechanism refers to cleft palate formation rather than cleft lip. Therefore, this answer is incorrect.
FINAL VERDICT…
Cleft lip occurs when there is a failure in fusion between the maxillary prominences and medial nasal prominences, which normally occurs during weeks 6–7 of embryonic development. These structures are responsible for forming parts of the upper lip and nose. If they fail to fuse properly, it results in a gap or fissure in the upper lip that may extend into one or both nostrils (as seen in this case).
This failure leads to an isolated cleft lip or may be associated with a cleft palate if additional developmental processes are affected.
Thus, failure of fusion between these specific facial prominences explains why this infant presents with a unilateral cleft lip extending toward the nostril.
CORRECT ANSWER: D) Failed fusion of the maxillary and medial nasal prominences
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That’s it for question 102!
See ya tomorrow 👋