Master Gross Anatomy & Embryology
for USMLE Step 1

Core Concepts

Gross Anatomy:

• Neuroanatomy: Cranial nerves (nuclei, foramina, function), brainstem syndromes (e.g., Medial Medullary, Lateral Pontine/Wallenberg), spinal cord tracts (ascending/descending, somatotopy), cerebral arteries & stroke syndromes (MCA, ACA, PCA), CSF flow & hydrocephalus.
• Thorax: Mediastinal compartments, heart chambers & valves (murmurs), coronary arterial supply & venous drainage (anastomoses), lung lobes/bronchopulmonary segments, diaphragm (innervation: C3-C5 phrenic).
• Abdomen: Foregut/midgut/hindgut derivatives & arterial supply (celiac, SMA, IMA), portal system & portacaval anastomoses, retroperitoneal organs (SAD PUCKER), inguinal canal (direct vs. indirect hernias), kidney anatomy & ascent.
• Pelvis & Perineum: Pelvic floor, bladder, reproductive organs, pudendal nerve distribution, anorectal anatomy.
• Musculoskeletal: Brachial plexus (roots, trunks, divisions, cords, branches) & associated lesions (Erb-Duchenne, Klumpke), nerve entrapment syndromes (carpal tunnel, ulnar, radial), major arterial & venous pathways, compartment syndrome.
• Head & Neck: Pharyngeal arches/pouches derivatives, salivary glands, thyroid/parathyroid, laryngeal innervation, cranial nerve foramina.

• Early Development: Fertilization, cleavage, gastrulation (ectoderm, mesoderm, endoderm derivatives), neural tube formation (primary vs. secondary neurulation), somite derivatives.
• Pharyngeal Arches/Pouches: Derivatives of skeletal, muscular, nervous, and glandular structures (e.g., 1st Arch: CN V, muscles of mastication; 2nd Arch: CN VII, muscles of facial expression; 3rd Arch: CN IX, stylopharyngeus; 4th/6th Arches: CN X, laryngeal muscles; Pouch derivatives: Ear, tonsils, thymus, parathyroids).
• Cardiovascular: Fetal circulation, septation of atria/ventricles, aortic arch derivatives (3rd: common/internal carotid; 4th: aortic arch/subclavian; 6th: pulmonary/ductus arteriosus).
• Gastrointestinal: Midgut rotation (physiologic herniation), diaphragm formation, foregut/midgut/hindgut anomalies (e.g., Meckel's diverticulum, Hirschsprung's).
• Urogenital: Kidney development (pronephros, mesonephros, metanephros), ascent & rotation, gonadal differentiation (Mullerian vs. Wolffian ducts).
• Teratology: Critical periods of development, common teratogens & their associated malformations (e.g., alcohol, thalidomide, ACEi, warfarin, phenytoin, valproate, isotretinoin, rubella, toxoplasmosis).

Clinical Presentation

• Neural Tube Defects: Anencephaly, spina bifida (myelomeningocele, meningocele, occulta) presenting as sacral mass, paralysis, sensory deficits.
• Congenital Heart Defects: Cyanosis (TOF, TGA, HLHS), heart murmurs (VSD, ASD, PDA), heart failure symptoms in infancy.
• GI Anomalies: Neonatal vomiting (pyloric stenosis, duodenal atresia), failure to pass meconium (Hirschsprung's), painless rectal bleeding (Meckel's), abdominal wall defects (omphalocele, gastroschisis).
• Pharyngeal Arch Syndromes: Craniofacial dysmorphology (e.g., Treacher Collins, DiGeorge - cardiac defects, hypocalcemia, immunodeficiency).
• Nerve Injuries: Specific motor/sensory deficits, muscle atrophy, foot drop (common fibular), wrist drop (radial), winged scapula (long thoracic).
• Hernias: Bulge in groin or umbilical region, pain, potential for incarceration/strangulation (abdominal pain, vomiting).
• Vascular Anomalies: Coarctation of aorta (HTN in upper, hypotension in lower extremities), AV malformations.
• Hydrocephalus: Enlarging head circumference, bulging fontanelle, sunsetting eyes in infants; headache, nausea, visual changes in older children/adults.

Diagnosis (Gold Standard)

Diagnosis for anatomical and embryological conditions often relies on a combination of clinical examination, imaging, and sometimes genetic testing.

• Prenatal Ultrasound: Screening for neural tube defects, congenital heart defects, renal anomalies, abdominal wall defects, and craniofacial abnormalities.
• Fetal MRI: For more detailed evaluation of anomalies identified on ultrasound.
• Echocardiogram: For confirmation and characterization of congenital heart disease (fetal or postnatal).
• CT/MRI: For detailed assessment of brain, spinal cord, abdominal, and musculoskeletal pathologies (e.g., stroke, spinal cord lesions, tumors, complex skeletal anomalies).
• Barium Studies: For GI anomalies like Hirschsprung's disease (barium enema showing transition zone) or malrotation.
• Angiography: For vascular anomalies (e.g., coarctation of aorta, AV malformations).
• Genetic Testing (Amniocentesis/CVS): When chromosomal anomalies or specific genetic syndromes are suspected due to multiple malformations.
• Clinical Neurological Exam: For assessing cranial nerve palsies, spinal cord lesions, or peripheral nerve injuries.

Management (First Line)

Management for anatomical and embryological conditions is often surgical, supportive, or preventative.

• Surgical Correction: For most significant congenital anomalies (e.g., neural tube defects, congenital heart defects, cleft lip/palate, tracheoesophageal fistula, abdominal wall defects, Hirschsprung's disease, hernia repair).
• Folate Supplementation: Periconceptional folic acid supplementation for prevention of neural tube defects.
• Supportive Care: For complex congenital syndromes (e.g., respiratory support for severe lung hypoplasia, nutritional support).
• Pharmacological Management:
  • Prostaglandin E1: To maintain patency of ductus arteriosus in ductal-dependent congenital heart lesions (e.g., severe coarctation, pulmonary atresia).
  • Indomethacin: To close a patent ductus arteriosus in premature infants.
• Rehabilitation: For neurological deficits resulting from spinal cord injuries, stroke, or severe peripheral nerve damage.
• Shunt Placement: For hydrocephalus (ventriculoperitoneal shunt).
• Avoidance of Teratogens: Counseling pregnant women on substances harmful to fetal development.

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