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DEPARTMENT OF ANATOMY - PAPER II: COMPLETE ANSWERS

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QUESTION 1: CASE STUDY (Ureteric Colic)

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A. Anatomical Basis of Pain Radiating from Back to Thigh (3 marks)

• Upper ureter (renal pelvis to pelvic brim): T10-L1 segments via the renal, aortic, and superior hypogastric plexuses
• Lower ureter (pelvic part): S2-S4 via the inferior hypogastric plexus

• The pain starts in the loin/back because the ureter receives sympathetic pain fibres entering the spinal cord at T10-L2 levels. The kidney/upper ureter shares innervation with the skin of the loin at these dermatomes.
• Radiation to the inner thigh: The genitofemoral nerve (L1, L2) supplies both the lower ureter (via the genital branch) and the skin of the inner upper thigh (via the femoral branch). When a calculus impacts the lower ureter near the pelvic brim, nociceptive signals travel via this nerve, producing referred pain in the inguinal region and inner thigh.
• Radiation to the testis/labia: The genital branch of genitofemoral nerve also supplies the cremaster/labium majus, explaining scrotal/labial pain.

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B. Three Constrictions of the Ureter & Clinical Importance (4 marks)

1. These are the sites where renal calculi most commonly lodge, producing ureteric colic
2. PUJ obstruction - most common site; causes hydronephrosis; managed by pyeloplasty
3. Pelvic brim impaction - pain referred to inguinal region and inner thigh (genitofemoral nerve territory)
4. UVJ obstruction - most common overall site of stone impaction; produces strangury (painful, frequent urge to urinate); managed by ureteroscopy or ESWL
5. Radiologically: calculi are most visible at these three narrowings on KUB (kidney-ureter-bladder) X-ray

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C. Posterior Relations of the Kidneys & Renal Angle (5 marks)

Structures above the 12th rib (both kidneys):

• Diaphragm (and through it, the pleura - pleural reflection)
• Subcostal nerve (T12)

Structures below the 12th rib:

• Transversus abdominis
• Quadratus lumborum
• Psoas major (medially)

• Subcostal nerve (T12)
• Iliohypogastric nerve (L1)
• Ilioinguinal nerve (L1)
• Genitofemoral nerve (L1, L2) - lies on psoas

• The right kidney is slightly lower; 12th rib crosses its upper third
• The left kidney is slightly higher; 12th rib crosses its middle third
• The pleural reflection (costodiaphragmatic recess) lies behind the upper pole of both kidneys - hence perinephric abscess can cause pleural effusion

Renal Angle (Costovertebral Angle - CVA):

• Defined as the angle between the 12th rib and the lateral border of the erector spinae (sacrospinalis) muscle
• It overlies the renal pelvis and upper ureter
• Clinical importance: Tenderness at the renal angle (CVA tenderness / Murphy's kidney punch) indicates:
  • Pyelonephritis
  • Perinephric abscess
  • Renal calculi
  • Hydronephrosis

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D. Congenital Anomalies of the Urinary System (3 marks)

A. Anomalies of Number:

• Renal agenesis - unilateral (1:1000; compensatory hypertrophy of other kidney) or bilateral (Potter's syndrome - lethal; oligohydramnios, pulmonary hypoplasia, characteristic facies)
• Supernumerary kidney - rare; extra kidney with separate collecting system

B. Anomalies of Position:

• Ectopic kidney - pelvic, iliac, thoracic; prone to hydronephrosis and infection
• Crossed ectopia - both kidneys on the same side

C. Anomalies of Fusion:

• Horseshoe kidney - most common fusion anomaly (1:400); lower poles fused across midline by isthmus of parenchyma; lies at L3-L4; isthmus trapped by inferior mesenteric artery; prone to infection, stone, and obstruction
• Pelvic cake kidney - complete fusion

D. Anomalies of Rotation:

• Malrotation - renal pelvis faces anteriorly instead of medially

E. Anomalies of the Collecting System:

• Duplex kidney/ureter (duplex collecting system) - double pelvis and ureter; upper moiety ureter opens ectopically (Weigert-Meyer rule)
• Pelvi-ureteric junction (PUJ) obstruction - most common obstructive uropathy in children
• Vesico-ureteric reflux (VUR) - retrograde flow of urine from bladder to ureter
• Megaureter - primary or secondary ureteral dilatation

F. Anomalies of the Bladder:

• Exstrophy of bladder - failure of mesoderm to invade infraumbilical region; bladder mucosa exposed on abdominal surface
• Patent urachus - failure of urachus to obliterate; urine drips from umbilicus

G. Anomalies of the Urethra:

• Hypospadias - external urethral meatus on ventral surface (most common urethral anomaly in males)
• Epispadias - meatus on dorsal surface (associated with exstrophy)
• Posterior urethral valves (PUV) - most common cause of severe obstructive uropathy in male neonates

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QUESTION 2: Knee Joint

(a) Type of Joint & Articular Surfaces (3 marks)

1. Medial tibiofemoral joint (between medial femoral condyle and medial tibial condyle)
2. Lateral tibiofemoral joint (between lateral femoral condyle and lateral tibial condyle)
3. Patellofemoral joint (between patella and patellar surface of femur)

• Femur: Articular surface covers both femoral condyles (medial and lateral) anteriorly, inferiorly, and posteriorly, plus the patellar (trochlear) groove anteriorly
• Tibia: Superior surface of medial and lateral tibial condyles (tibial plateau); medial tibial condyle is oval; lateral is circular; separated by intercondylar area (non-articular)
• Patella: Posterior (articular) surface - medial and lateral facets covered by the thickest hyaline cartilage in the body (up to 6 mm)
• Fibrocartilaginous menisci: Sit between femur and tibia; medial meniscus is C-shaped; lateral meniscus is nearly circular. They deepen the shallow tibial articular surface and improve congruence.

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(b) Movements at the Knee Joint (6 marks)

Movements:

1. Flexion: 0-135° (140° in full passive flexion); produced by hamstrings (biceps femoris, semitendinosus, semimembranosus), gastrocnemius, gracilis, sartorius, popliteus
2. Extension: 135° back to 0°; produced by quadriceps femoris (rectus femoris + 3 vasti) via patellar ligament
3. Medial rotation of leg (when knee flexed): popliteus, semitendinosus, semimembranosus, gracilis, sartorius
4. Lateral rotation of leg (when knee flexed): biceps femoris

Locking of the Knee (Screw Home Mechanism):

• Near the end of full extension, the femur medially rotates on the fixed tibia (in standing) - or the tibia laterally rotates on the femur (in sitting)
• This is because the medial femoral condyle is longer than the lateral; the lateral condyle exhausts its articular surface first, and then the femur pivots medially around the medial condyle
• At full extension, the cruciate ligaments and collateral ligaments become taut
• The knee is "locked" (screwed home) - no rotation is possible; it is a very stable position requiring minimal muscular effort (important for standing)
• Locking brings the knee into a close-packed position with maximum articular congruence

Unlocking (Initiation of Flexion):

• Before flexion can begin, the knee must be unlocked
• This is achieved by the popliteus muscle ("the key that unlocks the knee")
• Popliteus laterally rotates the femur on the fixed tibia (or medially rotates the tibia on the femur)
• It unscrews the knee, allowing flexion to begin
• Nerve supply of popliteus: tibial nerve (L4, L5, S1)

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Ligaments of the Knee Joint:

Extracapsular Ligaments:

1. Patellar ligament (ligamentum patellae) - continuation of quadriceps tendon; from apex of patella to tibial tuberosity; central to knee mechanics
2. Medial (Tibial) Collateral Ligament (MCL/TCL) - broad, flat band; from medial epicondyle of femur to medial condyle of tibia; firmly attached to medial meniscus (making it prone to tearing in injuries); resists valgus stress
3. Lateral (Fibular) Collateral Ligament (LCL/FCL) - cord-like; from lateral epicondyle of femur to head of fibula; NOT attached to lateral meniscus; resists varus stress
4. Oblique popliteal ligament - expansion of semimembranosus; strengthens posterior capsule
5. Arcuate popliteal ligament - Y-shaped; strengthens posterolateral capsule
6. Iliotibial tract - contributes to lateral stability

Intracapsular (Intra-articular) Ligaments:

1. Anterior Cruciate Ligament (ACL):
   • Attachment: Anterior intercondylar area of tibia → Posterior part of medial surface of lateral femoral condyle
   • Function: Prevents anterior displacement of tibia on femur; prevents hyperextension; checks medial rotation of femur in locking
   • Clinical: Most commonly torn ligament; "anterior drawer test" positive
2. Posterior Cruciate Ligament (PCL):
   • Attachment: Posterior intercondylar area of tibia → Anterior part of lateral surface of medial femoral condyle
   • Function: Prevents posterior displacement of tibia on femur; strongest ligament of the knee
   • Clinical: "Posterior drawer test" / "Sag sign" positive when torn

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The Menisci:

• C-shaped (semicircular), larger
• Anterior horn: attached to anterior intercondylar area
• Posterior horn: attached to posterior intercondylar area
• Peripherally attached to MCL - this makes it less mobile and MORE prone to injury
• Blood supply: middle genicular artery (peripheral 1/3 only - "red zone")
• Inner 2/3: avascular ("white zone") - heals poorly

• Nearly circular (O-shaped), smaller
• NOT attached to LCL - more mobile, LESS prone to injury
• Has a posterior hiatus through which passes the popliteus tendon

1. Deepen tibial surface - improve congruence and load distribution
2. Shock absorption - transmit 50-70% of load in extension, 85% in flexion
3. Stabilization of the knee
4. Lubrication - distribute synovial fluid
5. Proprioception - nerve endings in peripheral zone

• Medial meniscus tear is far more common (due to MCL attachment and less mobility)
• "Unhappy triad of O'Donoghue" - ACL + MCL + medial meniscus tear (valgus + rotational injury)
• Torn meniscus presents with: joint line pain, McMurray's test positive, locking or giving way
• Avascular inner zone cannot heal spontaneously - requires surgery (meniscectomy or repair)

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QUESTION 3: SHORT NOTES

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A. Intraperitoneal Spaces Around the Liver - Hepatorenal Pouch (6 marks)

Spaces Around the Liver:

1. Right anterior subphrenic space: Between anterior surface of right lobe of liver and diaphragm
2. Left anterior subphrenic space: Between anterior surface of left lobe, stomach, and diaphragm (includes area over spleen)
3. Right posterior subphrenic space (Hepatorenal Pouch / Morison's Pouch): Most important
4. Left posterior subphrenic space (lesser sac / omental bursa)

Hepatorenal Pouch (Morison's Pouch) - In Detail:

• Anteriorly and superiorly: Liver (visceral surface of right lobe) and gallbladder
• Posteriorly and inferiorly: Right kidney, right suprarenal gland, hepatic flexure of colon, second part of duodenum
• On the right: Continuous with the right paracolic gutter
• On the left: Communicates with the lesser sac via the epiploic foramen (foramen of Winslow)

1. Collection of pus: The most common site for right subphrenic abscess post-surgery or perforated duodenal ulcer, ruptured appendix, or cholecystitis - pus flows from pelvis (when patient is supine) or from right paracolic gutter
2. Blood in haemoperitoneum: Blood from ruptured liver, gallbladder, or right kidney collects here first in supine patients
3. Peritoneal dialysis: Fluid can pool here
4. Bile peritonitis: After cholecystectomy
5. Drainage: Surgical drainage is done via a posterior subcostal or posterior approach
6. USS/CT: Fluid in hepatorenal pouch is an early sign of intraperitoneal free fluid on FAST ultrasound

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B. Thoraco-Abdominal Diaphragm: Development and Anomalies (6 marks)

Development of the Diaphragm:

• Septum transversum begins forming at week 4
• Pleuroperitoneal canals close at week 6-8 (right side closes first; left closes later - explaining why CDH is more common on the left)
• By week 8, the diaphragm is complete
• The phrenic nerve (C3,4,5 - "C3,4,5 keep the diaphragm alive") grows into the diaphragm from the cervical myotomes - explaining its cervical nerve supply despite thoracic/abdominal position

Congenital Anomalies:

1. Congenital Diaphragmatic Hernia (CDH) - Bochdalek Hernia:
   • Failure of the pleuroperitoneal membrane to close the posterior-lateral foramen of Bochdalek
   • Most common on the LEFT side (85-90%) because the right is protected by early liver development
   • Abdominal organs (stomach, intestines, spleen) herniate into left thoracic cavity
   • Causes pulmonary hypoplasia (lungs cannot develop normally in compressed thorax) - main cause of mortality
   • Presents at birth with: respiratory distress, scaphoid abdomen, barrel chest, displaced heart sounds
   • Treatment: Surgical repair after stabilization; ECMO may be required
2. Morgagni Hernia (Parasternal/Retrosternal Hernia):
   • Herniation through the anterior foramen of Morgagni (between sternal and costal origins of diaphragm) - failure of proper fusion of the sternal and costal portions
   • Mostly on the right side (more common) as the pericardium protects the left
   • Usually asymptomatic; discovered incidentally; contains transverse colon or omentum
   • Less severe than Bochdalek hernia
3. Eventration of Diaphragm:
   • Abnormal elevation of the entire or part of the diaphragm due to failure of muscular development (aplasia of muscle)
   • Occurs mostly on the left side
   • Abdominal organs bulge upwards but remain below diaphragm
   • Phrenic nerve is intact
4. Hiatus Hernia:
   • Herniation of stomach through the oesophageal hiatus (enlarged hiatus)
   • Sliding (Type I) - most common; GEJ slides into thorax
   • Rolling/Paraesophageal (Type II) - gastric fundus herniates alongside the oesophagus
5. Accessory Diaphragm / Bilateral Diaphragm - rare

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C. Pleura - Recesses and Clinical Significance (6 marks)

The Pleura:

• Visceral pleura: Covers lung surface; no pain fibres (autonomic only)
• Parietal pleura: Lines thoracic wall, mediastinum, and diaphragm; has somatic pain fibres (intercostal and phrenic nerves)

Subdivisions of Parietal Pleura:

• Costal pleura: Lines inner surface of ribs and intercostal spaces (supplied by intercostal nerves)
• Diaphragmatic pleura: Covers upper surface of diaphragm (central part supplied by phrenic nerve C3-C5; peripheral part by intercostal nerves)
• Mediastinal pleura: Lines lateral mediastinum (phrenic nerve)
• Cervical pleura (Cupola/Dome of pleura): Projects 2.5 cm above medial end of clavicle into root of neck; reinforced by Sibson's fascia (suprapleural membrane)

Pleural Recesses (Sinuses):

1. Costodiaphragmatic Recess (Phrenicocostal Sinus):
   • Most important and largest recess
   • Between the costal pleura and diaphragmatic pleura
   • Deepest point: posterolateral, at the level of 10th rib in MCL, 12th rib posteriorly
   • Depth: approximately 5 cm (filled by lung base during deep inspiration)
   • Clinical Significance:
     • Pleural effusion collects here first (lowest point when upright) - detectable by chest X-ray when >300 mL
     • Thoracocentesis (pleural tap): Needle inserted in the 9th intercostal space, MCL, above the lower rib (to avoid NVB)
     • Basal pneumonia/effusion: Appears as blunting of the costophrenic angle on CXR
     • Danger zone: Abdominal organs and liver can be injured if needle placed too low
2. Costomediastinal Recess:
   • Between costal and mediastinal pleura
   • Found anteriorly, near the cardiac notch (more prominent on left)
   • Filled by the lingula of left lung during deep inspiration
   • Smaller and less clinically significant
3. Vertebromediastinal Recess - small, posterior

Clinical Significance of Pleura:

1. Pleurisy (Pleuritis): Inflammation of parietal pleura; causes sharp, stabbing pain worsened by breathing; friction rub on auscultation
2. Pleural Effusion: Fluid between visceral and parietal layers; dullness to percussion; decreased breath sounds
3. Pneumothorax: Air in pleural cavity; tracheal deviation (tension pneumothorax); treated by chest drain insertion (2nd ICS, MCL or 5th ICS, MAL)
4. Empyema: Pus in pleural cavity; needs drainage
5. Cervical pleura vulnerability: Injury to subclavian vessels or brachial plexus can also cause pneumothorax
6. Referred pain: Diaphragmatic pleura irritation → pain referred to shoulder (C3-C5 via phrenic nerve); costal pleura irritation → chest wall pain
7. Intercostal drainage uses the costodiaphragmatic recess as the entry point

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D. Common Peroneal Nerve - Detail and Anatomical Basis of Injury (6 marks)

Common Peroneal (Common Fibular) Nerve:

1. Descends along the lateral border of the popliteal fossa, following the medial border of biceps femoris tendon
2. Passes behind the head of the fibula
3. Winds around the neck of the fibula (most vulnerable point)
4. Enters the peroneus longus muscle, where it divides into its two terminal branches

• Lateral sural cutaneous nerve: Supplies skin on lateral and posterior surface of upper leg
• Peroneal communicating nerve: Joins medial sural cutaneous nerve to form sural nerve

1. Superficial Peroneal (Fibular) Nerve:
   • Passes through peroneus longus and runs between the peronei and EDL
   • Motor: Peroneus longus and brevis (eversion)
   • Sensory: Lower lateral leg, dorsum of foot and toes (except 1st web space)
   • Damaged in: Ankle inversion sprains, compartment syndrome
2. Deep Peroneal (Fibular) Nerve:
   • Passes through EDL, runs on interosseous membrane with anterior tibial vessels
   • Motor: Tibialis anterior, EDL, EHL, EDL, peroneus tertius, EDB
   • Sensory: First web space (1st and 2nd toes - dorsal)
   • Damaged in: Anterior compartment syndrome ("pump bumps"), tight plaster casts

Anatomical Basis of Injury:

1. Superficial position at the neck of fibula: The nerve is subcutaneous here, with no muscle to protect it - directly beneath the skin over the fibular neck
2. Bony contact: The nerve wraps tightly around the neck of the fibula with minimal soft tissue padding
3. Common causes of injury:
   • Fracture of neck of fibula (most common traumatic cause)
   • Pressure palsy: Prolonged squatting, sitting cross-legged, plaster cast (habitual crossing of legs compresses the nerve)
   • Knee dislocation - traction injury
   • Traction injury in severe inversion sprains
   • Tumours around fibular neck (ganglion cyst)
   • Tight plaster of Paris or below-knee cast
   • Compartment syndrome of lateral compartment

Effects of Common Peroneal Nerve Injury:

• Motor loss: Foot drop (inability to dorsiflex foot), inability to evert foot, toe drop
• Sensory loss: Lateral leg, dorsum of foot and first web space
• Deformity: Foot drop (due to unopposed plantarflexors) - patient walks with high-stepping gait (steppage gait)
• No effect on ankle jerk (tibial nerve S1)
• Loss of knee jerk: Not affected (femoral nerve L2-L4)

Applied: Foot drop must be distinguished from L4/L5 root lesion (both cause foot drop, but nerve lesion has no back pain, no dermatomal sensory loss up the leg).

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E. Portal Vein - Course, Relations, Tributaries; Portal Hypertension (6 marks)

Portal Vein:

• Formed behind the neck of the pancreas at the level of L2 by the union of:
  • Superior mesenteric vein (SMV) + Splenic vein
  • (Inferior mesenteric vein drains into splenic vein or SMV junction)
• Length: 8 cm; Diameter: ~8 mm; Normal pressure: 5-10 mmHg (portal hypertension >12 mmHg)

• Passes upward and to the right behind the first part of the duodenum
• Enters the free edge of the lesser omentum (hepatoduodenal ligament)
• Ascends in the hepatoduodenal ligament to the porta hepatis
• At porta hepatis, divides into right and left branches entering the respective liver lobes

• Portal vein - posterior
• Hepatic artery proper - left anterior
• Common bile duct (CBD) - right anterior
• This arrangement is inside the free edge of lesser omentum; forms the anterior boundary of the epiploic foramen (Foramen of Winslow)

1. Superior mesenteric vein (at formation)
2. Splenic vein (at formation)
3. Inferior mesenteric vein - usually joins splenic vein
4. Left gastric (coronary) vein - joins portal vein directly (key in variceal bleeding)
5. Right gastric vein - joins portal vein
6. Cystic vein (from gallbladder)
7. Para-umbilical veins - through falciform ligament

Portal Hypertension:

• Pre-hepatic: Portal vein thrombosis, splenic vein thrombosis
• Intra-hepatic (most common): Liver cirrhosis (alcohol, viral hepatitis), schistosomiasis
• Post-hepatic: Budd-Chiari syndrome, constrictive pericarditis, IVC obstruction

Portosystemic Anastomoses (Sites of Collateral Formation):

1. Splenomegaly (hypersplenism → pancytopenia)
2. Ascites (low albumin + portal pressure + aldosterone)
3. Oesophageal varices - present as haematemesis; treated by banding/sclerotherapy/TIPSS
4. Caput medusae - dilated veins radiating from umbilicus
5. Haemorrhoids (worsened)
6. Hepatic encephalopathy (from portosystemic shunting of ammonia)

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QUESTION 4: VERY SHORT NOTES

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A. Inguinal Ligament and Its Modifications

• Formed by the folded-back lower free edge of the external oblique aponeurosis
• Extends from the anterior superior iliac spine (ASIS) to the pubic tubercle
• Length: 12-14 cm
• Forms the floor of the inguinal canal
• Its lower surface forms a groove along which the femoral vessels pass

1. Lacunar Ligament (Gimbernat's Ligament): Triangular expansion from the medial end of the inguinal ligament that fans out posteriorly to attach to the pectineal line (pecten pubis); forms the medial boundary of the femoral ring; strangulates in femoral hernia
2. Pectineal Ligament (Cooper's Ligament): Lateral extension of the lacunar ligament along the pectineal line to the iliopectineal eminence; used in Cooper's repair (McVay repair) of inguinal/femoral hernias
3. Reflected Part (Colles' Ligament): Some fibres from the medial end reflect upward to the linea alba - forms part of the posterior wall of the inguinal canal
4. Iliopectineal Arch: Lateral part of inguinal ligament fuses with the iliac fascia to form this arch, separating the muscular and vascular lacunae

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B. Which Side is Varicocele More Common and Why?

1. Angle of venous drainage: The left testicular vein drains at a RIGHT ANGLE into the left renal vein (forming a T-junction), which creates increased hydrostatic pressure and turbulence, impeding venous return.
   • In contrast, the right testicular vein drains at an OBLIQUE ANGLE directly into the IVC, which is a more favorable hemodynamic arrangement.
2. Longer left testicular vein: The left vein is longer (8-10 cm longer than right), increasing venous column pressure.
3. Compression by Sigmoid Colon: The sigmoid colon crosses the left testicular vein and can compress it (called the "nutcracker effect" in some sources).
4. "Nutcracker Phenomenon": The left renal vein is compressed between the superior mesenteric artery (anteriorly) and the aorta (posteriorly), raising pressure in the left renal vein and making it harder for the left testicular vein to drain.

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C. Microanatomy of the Pancreas

Exocrine Pancreas:

• Acinar cells (serous): Pyramid-shaped cells arranged in acini; apex faces the lumen; filled with zymogen granules containing digestive enzymes (amylase, lipase, trypsinogen, chymotrypsinogen, etc.) in the apical cytoplasm
• Centroacinar cells: Modified ductal cells that project into the acinus; secrete bicarbonate-rich fluid (stimulated by secretin)
• Ductal cells: Line interlobular and main pancreatic ducts; secrete water and electrolytes

• Basal region: Rich in rough ER (basophilic, due to enzyme synthesis) - "basal basophilia"
• Apical region: Zymogen granules (acidophilic/eosinophilic)
• Nuclei: basal position
• Staining: PAS positive (for glycoproteins in granules)

Endocrine Pancreas - Islets of Langerhans:

• Pale-staining clusters scattered throughout the parenchyma (most numerous in the tail)
• 1-2 million islets per pancreas
• Four main cell types:

• Rich blood supply (islets receive 10-15x more blood flow per gram than exocrine pancreas)
• No secretory ducts - hormones secreted directly into blood (endocrine)

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D. Cross-Section of Thorax at the Level of 5th Thoracic Vertebra (T5)

Bony Structures:

• Body of T5 vertebra posteriorly (with spinal cord within vertebral canal)
• 5th ribs laterally
• Body of sternum anteriorly (at this level - below sternal angle)

Mediastinal Structures at T5:

• Tracheal bifurcation (carina): Occurs at T4-T5 disc level (sternal angle = T4/5 junction); so at the T5 body itself, the two main bronchi are seen (right main bronchus angling more to the right)
• Ascending aorta / Aortic arch: The arch lies at the level of T4; by T5 level, the descending thoracic aorta is beginning, lying left of the vertebral column
• Superior vena cava (SVC): Formed at T4/T5 junction; seen on the right
• Azygos vein: Posterior to SVC on the right; drains into SVC at T4
• Oesophagus: Posterior to trachea/bronchi, anterior to vertebral column, lying just left of midline
• Thoracic duct: Lies between oesophagus and spine, to the right at this level (it crosses to the left at T5)
• Vagus nerves: Right (posterior to right bronchus); Left (anterior to aortic arch, now descending)
• Phrenic nerves: Lateral to pericardium on both sides

The Sternal Angle (Angle of Louis) at T4/5 is the key landmark marking:

• Tracheal bifurcation (carina)
• Arch of aorta begins and ends
• Azygos arch drains into SVC
• 2nd costal cartilage articulates with sternum
• Superior mediastinum ends / Inferior mediastinum begins
• The plane of the transverse thoracic plane

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E. Trisomy 21 (Down Syndrome)

1. Non-disjunction (94-95%): Failure of chromosome 21 to separate during meiosis I or II; usually maternal (87% maternal, 13% paternal); NOT hereditary; risk increases with maternal age
2. Translocation (4-5%): Chromosome 21 fuses with another chromosome (usually 14, 21, or 22 - Robertsonian translocation); not related to maternal age; can be hereditary (carrier parents)
3. Mosaicism (1-2%): Non-disjunction occurs during mitosis after fertilization; two cell lines (normal + trisomy 21); milder phenotype

• Age 20: 1 in 1500
• Age 35: 1 in 400
• Age 40: 1 in 100
• Age 45: 1 in 30

• Flat facial profile
• Upslanting palpebral fissures
• Epicanthal folds
• Brushfield spots (speckled iris)
• Small ears, flat nasal bridge
• Protruding tongue (relative macroglossia)

• Intellectual disability (IQ 25-70)
• Hypotonia ("floppy baby")
• Developmental delay

• Single palmar crease (Simian crease)
• Clinodactyly (incurved 5th finger)
• Short broad hands

• AVSD (Endocardial cushion defect) - most characteristic
• VSD, ASD, PDA

• Duodenal atresia ("double bubble sign" on X-ray)
• Hirschsprung disease
• Imperforate anus

• Atlantoaxial instability (C1-C2 subluxation risk)
• Acute megakaryoblastic leukaemia (AMKL/AML M7) - 10-20x increased risk; also ALL
• Alzheimer's disease (virtually all develop by age 40 - APP gene on chr. 21)
• Hypothyroidism
• Male infertility (most males are infertile); females may be fertile

• Prenatal: NIPT (non-invasive prenatal testing - cell-free fetal DNA); 1st trimester combined screening (nuchal translucency + β-hCG + PAPP-A); amniocentesis/CVS for karyotype
• Postnatal: Karyotype (FISH or conventional); clinical features

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