A 56-year-old man presents to the emergency room after being in a motor vehicle accident. He was driving on an icy road when his car swerved off the road and ran head on into a tree. He complains of severe pain in his right lower extremity. He denies loss of consciousness during the accident. His past medical history is notable for poorly controlled hypertension, hyperlipidemia, and major depressive disorder. He takes enalapril, atorvastatin, and sertraline. His temperature is 99.1°F (37.3°C), blood pressure is 155/85 mmHg, pulse is 110/min, and respirations are 20/min. On exam, he is alert and fully oriented. He is unable to move his right leg due to pain. Sensation is intact to light touch in the sural, saphenous, tibial, deep peroneal, and superficial peroneal distributions. His leg appears adducted, flexed, and internally rotated. An anteroposterior radiograph of his pelvis would most likely demonstrate which of the following findings?

Femoral head smaller than contralateral side and posterior to acetabulum

Fracture line extending between the greater and lesser trochanters

Femoral head larger than contralateral side and inferior to acetabulum

Fracture line extending through the femoral neck

Fracture line extending through the subtrochanteric region of the femur

An 11-year-old girl is brought to the office by her mother due to complaint of intermittent and severe periumbilical pain for 1 day. She does not have any significant past medical history. She provides a history of a recent school trip to the suburbs. On physical examination, there is a mild tenderness around the umbilicus without any distension or discharge. There is no rebound tenderness. Bowel sounds are normal. An abdominal imaging shows enlarged mesenteric lymph nodes, and she is diagnosed with mesenteric lymphadenitis. However, incidentally, a mass of tissue was seen joining the inferior pole of both kidneys as shown in the image. Which of the following best describes this renal anomaly?

Association with ureteropelvic junction obstruction (UPJO)

Fused kidneys ascend beyond superior mesenteric artery.

Rapid progression to acute renal failure

Kidneys are usually non-functional.

Increased risk of developing renal vein thrombosis

A newborn of a mother with poor antenatal care is found to have a larger than normal head circumference with bulging fontanelles. Physical examination reveals a predominant downward gaze with marked eyelid retraction and convergence-retraction nystagmus. Ultrasound examination showed dilated lateral ventricles and a dilated third ventricle. Further imaging studies reveal a solid mass in the pineal region. Which of the following is the most likely underlying pathophysiological mechanism responsible for the neurological signs in this patient?

Compression of periaqueductal grey matter

Stenotic intraventricular foramina

Hypertrophic arachnoid granulations

Normal lumbar puncture opening pressure

A 2-year-old child is brought to the emergency department with rapid breathing and a severe cyanotic appearance of his lips, fingers, and toes. He is known to have occasional episodes of mild cyanosis, especially when he is extremely agitated. This is the worst episode of this child’s life, according to his parents. He was born with an APGAR score of 8 via a normal vaginal delivery. His development is considered delayed compared to children of his age. History is significant for frequent squatting after strenuous activity. On auscultation, there is evidence of a systolic ejection murmur at the left sternal border. On examination, his oxygen saturation is 71%, blood pressure is 81/64 mm Hg, respirations are 42/min, pulse is 129/min, and temperature is 36.7°C (98.0°F). Which of the following will most likely be seen on chest x-ray (CXR)?

Increased pulmonary vascular markings

Cardiomegaly with globular heart

A 65-year-old man presents to the emergency department with abdominal pain and a pulsatile abdominal mass. Further examination of the mass shows that it is an abdominal aortic aneurysm. A computed tomography scan with contrast reveals an incidental finding of a horseshoe kidney, and the surgeon is informed of this finding prior to operating on the aneurysm. Which of the following may complicate the surgical approach in this patient?

Anomalous origins of multiple renal arteries

Low glomerular filtration rate due to unilateral renal agenesis

Proximity of the fused kidney to the celiac artery

Abnormal relationship between the kidney and the superior mesenteric artery

There are no additional complications

A 23-year-old woman presents to the emergency department with an acute exacerbation of her 3-month history of low back and right leg pain. She says she has had similar symptoms in the past, but this time the pain was so excruciating, it took her breath away. She describes the pain as severe, shock-like, and localized to her lower back and radiating straight down the back of her right thigh and to her calf, stopping at the ankle. Her pain is worse in the morning, and, sometimes, the pain wakes her up at night with severe buttock and posterior thigh pain but walking actually makes the pain subside somewhat. The patient reports no smoking history or alcohol or drug use. She has been working casually as a waitress and does find bending over tables a strain. She is afebrile, and her vital signs are within normal limits. On physical examination, her left straight leg raise test is severely limited and reproduces her buttock pain at 20° of hip flexion. Pain is worsened by the addition of ankle dorsiflexion. The sensation is intact. Her L4 and L5 reflexes are normal, but her S1 reflex is absent on the right side. A CT of the lumbar spine shows an L5–S1 disc protrusion with right S1 nerve root compression. Which of the following muscle-nerve complexes is involved in producing an S1 reflex?

Gastrocnemius/soleus-tibial nerve

Tibialis posterior-tibial nerve

Quadriceps femoris-femoral nerve

A 27-year-old female ultramarathon runner presents to the physician with complaints of persistent knee pain. She describes the pain to be located in the anterior area of her knee and is most aggravated when she performs steep descents down mountains, though the pain is present with running on flat roads, walking up and down stairs, and squatting. Which of the following would most likely be an additional finding in this patient’s physical examination?

Pain upon compression of the patella while the patient performs flexion and extension of the leg

Pain upon pressure placed on the lateral aspect of the knee

Pain upon pressure placed on the medial aspect of the knee

Excessive posterior displacement of the tibia

Excessive anterior displacement of the tibia

Anatomical variants in imaging for USMLE Step 3: High-Yield Anatomy Lessons

Introductory Concepts - What's Normal-ish?

Anatomical Variant: A common, asymptomatic deviation from typical anatomy. Crucial to differentiate from true pathology to avoid misdiagnosis and unnecessary procedures.
Key Features: Usually discovered incidentally. Characteristically have well-defined, smooth borders and follow the signal/density of their parent tissue on all imaging sequences.
Common Examples: Azygos lobe, accessory spleen, developmental venous anomaly (DVA), persistent fetal circulation patterns.

⭐ Many variants are classic "leave-me-alone" lesions. Confident identification requires no follow-up.

CNS Variants - Brainy Quirks

Cavum Septum Pellucidum (CSP) & Vergae (CSPV): Midline CSF-filled cavities. Represents a failure of fusion of the septal leaflets. Common in neonates, usually fuses post-birth. A frequent, benign incidental finding in adults.
Mega Cisterna Magna: Focal enlargement of the subarachnoid space in the posterior fossa (vertical dimension >10 mm on midline sagittal). Crucially, the cerebellum and 4th ventricle are morphologically normal.
Pineal Cysts: Common, typically asymptomatic, and small (<1 cm). Found incidentally on MRI.

⭐ The primary clinical task is to differentiate these benign variants from pathological lesions like arachnoid cysts, epidermoid cysts, or cystic neoplasms which may require intervention.

Thoracic Variants - Chest Oddities

Azygos Lobe: Not a true lobe; an anomalous azygos vein creates a fissure, isolating a portion of the right upper lobe. Mimics a bulla or mass on CXR.
Pectus Excavatum: Sternal depression ("funnel chest"). Can displace the heart, obscuring the right heart border.
Pectus Carinatum: Sternal protrusion ("pigeon breast").
Poland Syndrome: Unilateral absence of pectoralis major muscle, often with ipsilateral syndactyly and breast hypoplasia.

⭐ In Pectus Excavatum, the apparent cardiomegaly and obscured right heart border on PA chest X-ray are artifacts of cardiac compression and rotation, a phenomenon known as "pancake heart."

Abdominopelvic Variants - Guts & Glory

Situs Inversus: Mirror-image reversal of organs. Dextrocardia on CXR is a clue. Associated with primary ciliary dyskinesia (Kartagener syndrome).
Accessory Spleen (Splenule): Common incidentaloma (~10-30% of patients), typically near the splenic hilum. Can be mistaken for a tumor.
Horseshoe Kidney: Fused lower poles get trapped under the Inferior Mesenteric Artery (IMA) during ascent. ↑ risk of stones & infection.

⭐ Meckel's Diverticulum: Most common GI congenital anomaly. 📌 Rule of 2s: 2% prevalence, 2 feet from ileocecal valve, 2 inches long, 2% symptomatic, presents by age 2.

Musculoskeletal Variants - Bony Bonuses

Accessory Ossicles: Unfused bone centers that mimic fractures. Differentiated by smooth, corticated borders.
- Os Trigonum: Posterior ankle. Pain with plantar flexion (os trigonum syndrome).
- Accessory Navicular: Medial foot. Can lead to painful tendinopathy.
Common Variants:
- Fabella: Sesamoid in lateral gastrocnemius tendon.
- Bipartite Patella: Unfused superolateral patellar pole. Often bilateral.

⭐ The fabella, a sesamoid bone in the gastrocnemius tendon, is present in up to 30% of individuals and can cause posterolateral knee pain.

Foot X-rays: Accessory ossicles and anatomical variants

High‑Yield Points - ⚡ Biggest Takeaways

Anatomical variants are common incidental findings, not true pathologies; recognition prevents misdiagnosis.

An azygos lobe is not a true accessory lobe but a fissure created by an aberrant azygos vein.

Accessory spleens (splenules) are frequent and can mimic lymphadenopathy or pancreatic/adrenal tumors.

A horseshoe kidney is the most common renal fusion anomaly, increasing risk for UPJ obstruction and stones.

A persistent left-sided IVC can be mistaken for para-aortic lymphadenopathy.

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