A 35-year-old woman, gravida 2 para 1, comes to the office for prenatal evaluation.  The patient says she had been using condoms for birth control and unintentionally became pregnant.  Her last menstrual period was 2 months ago, and home pregnancy tests are positive.  She has not been taking prenatal vitamins.  The patient has a healthy, 5-year-old daughter who was born after an uneventful pregnancy.  She has a history of rheumatoid arthritis, and her joint symptoms are controlled with methotrexate therapy.  The patient has no other medical conditions and takes no other medications.  She does not use tobacco, alcohol, or illicit drugs.  Vital signs are within normal limits, and physical examination shows no abnormalities.  Serum β-hCG is elevated, and transvaginal ultrasonography shows a normal gestational sac and embryo.  This patient’s unborn child is at the greatest risk for which of the following congenital defects?

A 38-year-old woman comes to the emergency department due to severe abdominal pain and delivers a stillborn neonate who is missing a large portion of the scalp and skull, partially exposing the brain, which is also only partially formed.  The patient did not know she was pregnant and had no prenatal care.  Medical history is significant for bipolar disorder and chronic hypertension.  Interview of the patient reveals pressured and tangential speech, and she states that she takes certain prescription medications when she remembers but cannot identify which ones.  If used during pregnancy, which of the following substances most likely contributed to this neonate’s abnormalities?

A 35-year-old woman, gravida 2 para 1, comes to the office for a routine prenatal visit.  Her first child was born with a hydrocele and syndactyly of the first and second toes.  The patient has no significant medical conditions and takes no medications except for a multivitamin.  Examination shows clear lungs and normal S1 and S2 heart sounds.  The abdomen is soft and nontender.  Uterine size, fetal movements, and fetal cardiac activity are all within normal limits.  The patient undergoes an amniocentesis at 18 weeks gestation.  Amniotic fluid analysis shows an increased level of acetylcholinesterase.  This patient’s amniocentesis results most likely suggest failure of which of the following processes?

A 16-year-old obese primigravida girl comes to the emergency department in active labor.  In an attempt to conceal the pregnancy from her family, the patient did not receive prenatal care or take prenatal vitamins.  A fetal heartbeat is undetectable, and she delivers a stillborn boy via vaginal delivery.  Examination of the stillborn shows several dysmorphic features including closely set eyes and a midline mass consistent with a proboscis.  A fetal autopsy reveals fused cerebral hemispheres with an absent forebrain fissure and a single intracranial ventricle.  Which of the following is the most likely mechanism for these findings?

A 2-year-old boy who recently immigrated to the United States with his family is brought to the office for irritability and excessive crying.  The mother’s pregnancy was uneventful, but the patient was delivered via cesarean section due to obstructed labor.  His mother reports that he has developed more slowly than his brother in ability to lift his head, crawl, and walk.  She has also noticed that the patient’s head is larger than other children of the same age.  His weight is at the 60th percentile, height is at the 50th percentile, and head circumference is at the 95th percentile.  CT scan of the head is shown in the image below.

Which of the following is the most likely diagnosis?

A 21-year-old woman comes to the office due to a long history of episodic headaches, dizziness, and gait imbalance.  The headaches are located in the occipital region, are characterized as dull, and last for several hours.  Physical examination reveals gait ataxia but no focal weakness or sensory loss.  MRI of the brain shows low-lying cerebellar tonsils extending below the plane of the foramen magnum into the vertebral canal.  Which of the following is the most likely cause of this patient’s condition?

A 38-year-old woman is brought to the emergency department after a motor vehicle collision.  She sustained no injuries in the collision but is adamant she did not see the car that hit her along the front side of her vehicle.  The patient also reports that she has been having daily headaches and has not menstruated in the last 4 months.  On physical examination, there is bitemporal hemianopia but no other abnormalities.  MRI of the brain is shown below.

This patient’s lesion most likely originates from which of the following embryologic layers?