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AN79.1-6 | 3rd to 8th week of development — Practice Quiz
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The primitive streak first appears in which part of the embryonic disc during the 3rd week?
Correct! The primitive streak appears at Day 14–15 in the CAUDAL MIDLINE of the epiblast. It is the first landmark of the 3rd week and establishes the cranial-caudal, left-right, and dorsal-ventral axes.
Primitive streak = caudal midline epiblast; Day 14–15; establishes body axes. Cells migrate through the streak to form the intraembryonic mesoderm (gastrulation = bilaminar → trilaminar disc).
Incorrect. The primitive streak arises in the CAUDAL midline of the EPIBLAST (not cranial, not hypoblast, not lateral mesoderm). It marks the beginning of gastrulation.
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A neonate is born with a large sacrococcygeal mass containing hair, cartilage, and neural tissue. The embryological origin of this tumour is remnants of which structure?
Correct! Sacrococcygeal teratoma arises from incompletely regressed PRIMITIVE STREAK remnants. Pluripotent epiblast cells persist in the presacral region and form derivatives of all three germ layers (hence hair = ectoderm, cartilage = mesoderm, neural tissue = ectoderm).
Primitive streak normally regresses completely by end of 4th week. Persistence → sacrococcygeal teratoma. Contains all 3 germ layer derivatives. Must be excised soon after birth — 80% benign at birth but risk of malignancy rises with delay.
Incorrect. Sacrococcygeal teratoma = PRIMITIVE STREAK remnants (not notochord, which gives chordoma; not neural tube).
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The notochord persists in the adult as which specific structure within the vertebral column?
Correct! The notochord DEGENERATES within the vertebral bodies but PERSISTS and EXPANDS at the intervertebral disc levels as the NUCLEUS PULPOSUS — the soft, gelatinous central core of the disc.
Nucleus pulposus = notochordal remnant. High water content → decreases with age. With aging, annulus fibrosus tears → nucleus pulposus herniates → disc prolapse → nerve root compression → sciatica. Most common at L4–L5 and L5–S1.
Incorrect. The NUCLEUS PULPOSUS (central gel core of the disc) is the notochordal remnant. The annulus fibrosus comes from sclerotome mesenchyme. The ligaments are separate structures.
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The primary inductive role of the notochord is to induce the overlying ectoderm to differentiate into which structure?
Correct! The notochord induces the overlying ECTODERM to thicken and form the NEURAL PLATE — the precursor of the entire central nervous system via neurulation.
Notochord → induces ectoderm → neural plate → neural folds → neural tube (CNS). This is the key embryological signalling cascade of the 3rd week. Without the notochord, the neural plate does not form.
Incorrect. The notochord's primary inductive signal causes the overlying ectoderm to form the NEURAL PLATE. This process is called "neural induction" and is the first step of neurulation.
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Neural tube closure begins at the mid-cervical region on Day 22. The posterior neuropore — the last point to close — closes at approximately which gestational day?
Correct! The POSTERIOR NEUROPORE closes on Day 27. The ANTERIOR neuropore closes on Day 25. Closure starts mid-cervically and proceeds both cranially and caudally simultaneously.
Neural tube closure sequence: mid-cervical (Day 22) → anterior neuropore (Day 25) → posterior neuropore (Day 27). NTD prevention: periconceptional folic acid (0.4–5 mg/day).
Incorrect. Posterior neuropore closes Day 27; anterior neuropore Day 25. If the ANTERIOR neuropore fails to close → ANENCEPHALY. If the POSTERIOR neuropore fails to close → SPINA BIFIDA / myelomeningocele.
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A 22-week fetus is found on anomaly scan to have: absence of the cranial vault and exposed brain tissue with no overlying skin. The "lemon sign" is absent (this is anencephaly, not myelomeningocele); the brain is not formed above the midbrain. This is most consistent with failure of which structure to close?
Correct! ANENCEPHALY results from failure of the ANTERIOR NEUROPORE to close on Day 25. The brain above the midbrain does not form; the cranial vault (calvaria) is absent. The fetus has a "frog-like" appearance. Incompatible with prolonged life.
Anterior neuropore (Day 25) → if fails to close = ANENCEPHALY (no brain, no calvaria). Posterior neuropore (Day 27) → if fails = spina bifida at lumbosacral level. AFP elevated in both (leaks into amniotic fluid).
Incorrect. ANENCEPHALY = ANTERIOR neuropore failure. Posterior neuropore failure → spina bifida (myelomeningocele). Pleuroperitoneal canal = diaphragm/CDH.
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Which component of the somite gives rise to the vertebral bodies, vertebral arches, and intervertebral disc annulus fibrosus?
Correct! The SCLEROTOME (ventromedial somite) migrates around the notochord and neural tube to form the vertebral bodies, arches, and annulus fibrosus. Myotome → skeletal muscle. Dermatome → back dermis.
Somite 3 derivatives mnemonic — "ScMyDe": Sclerotome = skeleton, Myotome = muscles, Dermatome = dermis. Sclerotome resegments: each vertebra = caudal half of one sclerotome + cranial half of next → muscles/nerves cross joints.
Incorrect. SCLEROTOME → vertebral column + ribs. Myotome → trunk skeletal muscles. Dermatome → dermis of back. Lateral plate mesoderm → body wall, visceral mesoderm, coelom.
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A neonate presents at birth with respiratory distress, scaphoid abdomen, and heart shifted to the right on chest X-ray. Loops of bowel are seen in the left pleural cavity. This condition results from failure of which embryological structure to close?
Correct! Congenital Diaphragmatic Hernia (CDH) results from failure of the PLEUROPERITONEAL CANAL to close — most commonly on the LEFT side (posterior foramen of Bochdalek). Abdominal contents herniate into the pleural cavity → pulmonary hypoplasia → severe respiratory distress at birth.
CDH = pleuroperitoneal canal fails to close (usually LEFT, Bochdalek). Abdominal organs in chest → lung hypoplasia → neonatal respiratory failure. Pleuropericardial fold closure → separates pericardial from pleural cavity (its failure → different defect).
Incorrect. CDH = LEFT PLEUROPERITONEAL CANAL failure (Bochdalek hernia). The pleuropericardial fold separates the pericardial and pleural cavities (a different structure).
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A 24-year-old woman with epilepsy was taking sodium valproate for seizure control and conceives an unplanned pregnancy. She took valproate throughout the 1st and 2nd trimester. Which trimester and which type of defect does valproate most critically cause?
Correct! Valproate is a FOLATE ANTAGONIST and neural tube teratogen. Its critical window is the 1ST TRIMESTER (3rd–8th weeks of organogenesis, specifically Days 22–27 for neural tube closure). It causes neural tube defects (myelomeningocele, spina bifida) and cardiac defects. It also causes later neurodevelopmental effects but the STRUCTURAL defect is 1st trimester.
Critical period of organogenesis = 3rd–8th weeks. Teratogens cause STRUCTURAL defects only in this window. Valproate = NTD (neural tube), cardiac defects. Folic acid 5 mg/day is mandatory for epileptic women on valproate planning pregnancy.
Incorrect. Valproate causes STRUCTURAL defects in the 1st trimester (organogenesis period), specifically NTDs. The critical window = 3rd–8th weeks. Women of childbearing age on valproate must receive folic acid 5 mg/day and be counselled about teratogenic risk.
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A 28-year-old woman planning her second pregnancy had a first child with myelomeningocele. She comes to the antenatal clinic for preconception counselling. What is the recommended periconceptional folic acid dose for her compared with a woman with no prior NTD-affected pregnancy?
Correct! HIGH-RISK women (previous NTD-affected child, maternal diabetes, antiepileptic drugs, obesity) should receive FOLIC ACID 5 mg/day periconceptionally. LOW-RISK women (no prior NTD) receive 0.4 mg/day. Both should start at least 1 month before conception and continue through the 1st trimester.
Folic acid doses: 0.4 mg/day (routine; no prior NTD) vs 5 mg/day (prior NTD, diabetes, antiepileptics, BMI >30). Start ≥1 month before conception; continue through 1st trimester (neural tube closes by Day 27 of 3rd week).
Incorrect. The dose is HIGHER for high-risk women: 5 mg/day if previous NTD child vs 0.4 mg/day for routine supplementation. This distinction is clinically important and tested frequently in Indian exams.
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