⚡ Anatomy · USMLE Step 1

Pharyngeal arches (derivatives)

• Arch 1 (CN V): mastication muscles, mylohyoid, anterior digastric, tensor tympani; maxilla, mandible, incus, malleus.
• Arch 2 (CN VII): facial expression, posterior digastric, stylohyoid, stapedius; stapes, styloid, lesser horn hyoid.
• Arch 3 (CN IX): stylopharyngeus; greater horn hyoid; common & internal carotids.
• Arch 4 (CN X superior laryngeal): cricothyroid, soft palate; thyroid cartilage; aortic arch (left).
• Arch 6 (CN X recurrent laryngeal): intrinsic laryngeal muscles (except cricothyroid); laryngeal cartilages; pulmonary arteries.

Carotid & subclavian

• ECA branches: superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, superficial temporal, maxillary (→ middle meningeal).
• Subclavian: internal thoracic, vertebral, costocervical, thyrocervical.

Thyroid & tongue

• Thyroid from foramen cecum, migrates caudally. Thyroglossal duct cyst moves with swallowing.
• Tongue: ant ⅔ – sensation V₃, taste VII; post ⅓ – IX; motor XII (except palatoglossus X).

Heart & pericardium

• Pericardial sinuses: transverse (posterior to aorta/pulmonary trunk) and oblique (posterior to left atrium).
• Coronary dominance: RCA supplies SA node (60%), AV node (80%), posterior IV septum.
• LAD (left anterior descending) – most common MI (50%).

Conduction

• SA node (RCA) → AV node (RCA) → bundle of His (LAD) → Purkinje.
• Moderator band carries right bundle branch.

Lungs & pleura

• Right lung: 3 lobes (horizontal + oblique fissures). Left: 2 lobes (oblique fissure).
• Pleural recesses: costodiaphragmatic & costomediastinal.
• Innervation: parietal pleura – intercostal (local pain) & phrenic (referred shoulder); visceral – autonomic.

Histology

• Type II pneumocytes → surfactant (lamellar bodies).
• Clara cells – bronchiolar secretory, stem cells, detox.
• Kulchitsky cells – neuroendocrine, carcinoid tumors.

Arterial supply

• Celiac (foregut): left gastric, splenic, common hepatic.
• SMA (midgut): inferior pancreaticoduodenal, intestinal arteries, ileocolic, right/middle colic.
• IMA (hindgut): left colic, sigmoid, superior rectal.

Portal system

• Portal vein = SMV + splenic vein. IMV → splenic vein.
• Portacaval anastomoses: esophageal varices, hemorrhoids, caput medusae.

GI histology

• Stomach: parietal (HCl + intrinsic factor), chief (pepsinogen).
• Duodenum: Brunner’s glands (submucosa, alkaline mucus).
• Ileum: Peyer’s patches, M cells.
• Large intestine: no villi, numerous goblet cells, teniae coli.

Kidney & ureter

• Ureter constrictions: PUJ, pelvic brim, intramural – stone lodgment.
• Horseshoe kidney – fused lower poles, trapped under IMA.

Female reproductive

• Ovary: cortex (follicles) + medulla. Corpus luteum → progesterone; degenerates → corpus albicans.
• Oviduct: ampulla (fertilization), ciliated epithelium.
• Uterus: functionalis (sloughed) + basalis (regenerative).
• Ectopic pregnancy – ampulla most common.

Male reproductive

• Sertoli cells: blood‑testis barrier, inhibin, ABP (FSH).
• Leydig cells: testosterone (LH).
• Prostate: transition zone (BPH), peripheral zone (cancer).

Perineum

• Superficial pouch: crura, bulb, Bartholin glands.
• Deep pouch: sphincter urethrae, Cowper glands.
• Pudendal nerve (S2–S4) – block at ischial spine.

Brachial plexus

• Erb-Duchenne (C5–C6): waiter’s tip – loss of abductors/lateral rotators, biceps/brachialis.
• Klumpke (C8–T1): claw hand (ulnar) + ape hand (median), Horner possible.

Rotator cuff (SITS)

• Supraspinatus, Infraspinatus, Teres minor, Subscapularis.
• Supraspinatus – most commonly torn.

Carpal tunnel

• Contents: median nerve + 9 flexor tendons.
• Scaphoid fracture – most common, risk of AVN.
• Hook of hamate – may injure ulnar nerve.

Lumbosacral plexus

• Femoral (L2–L4) – knee extension, hip flexion.
• Obturator (L2–L4) – thigh adduction.
• Tibial (L4–S3) – plantar flexion, inversion.
• Common fibular (L4–S2) – dorsiflexion, eversion.

Nerve injuries & gait

• Superior gluteal → Trendelenburg gait (pelvis drops).
• Sciatic – weak knee flexion, all below‑knee functions lost.
• Deep fibular – sensory loss in first web space.

Knee & ankle

• Unhappy triad: tibial collateral + medial meniscus + ACL.
• Ankle inversion → anterior talofibular ligament (most common).

Vertebral column

• Spinal cord ends at L1–L2; dural sac at S2.
• Disk herniation – most common posterolateral; nerve root affected one level below.
• Lumbar puncture – L4–L5, needle passes through ligamentum flavum → epidural → dura → arachnoid.

Autonomic nervous system

• Sympathetic (T1–L2): preganglionic short, postganglionic long; NE.
• Parasympathetic (CN III,VII,IX,X & S2–S4): preganglionic long, postganglionic short; ACh.
• Horner syndrome – ipsilateral ptosis, miosis, anhydrosis (T1–T2 lesion).