A 57-year-old man presents to his family physician for a routine exam. He feels well and reports no new complaints since his visit last year. Last year, he had a colonoscopy which showed no polyps, a low dose chest computerized tomography (CT) scan that showed no masses, and routine labs which showed a fasting glucose of 93 mg/dL. He is relatively sedentary and has a body mass index (BMI) of 24 kg/m^2. He has a history of using methamphetamines, alcohol (4-5 drinks per day since age 30), and tobacco (1 pack per day since age 18), but he joined Alcoholics Anonymous and has been in recovery, not using any of these for the past 7 years. Which of the following is indicated at this time?

Chest computerized tomography (CT) scan

A 36-year-old male is taken to the emergency room after jumping from a building. Bilateral fractures to the femur were stabilized at the scene by emergency medical technicians. The patient is lucid upon questioning and his vitals are stable. Pain only at his hips was elicited. Cervical exam was not performed. What is the best imaging study for this patient?

Computed tomography (CT) scan of cervical spine, hips, and lumbar area

AP and lateral radiographs of hips

Lateral radiograph (x-ray) of hips

Magnetic resonance imaging (MRI) of hips, knees, lumbar, and cervical area

Anterior-posterior (AP) and lateral radiographs of hips, knees, lumbar, and cervical area

A cardiologist is studying how a new virus that infects the heart affects the electrical conduction system of the cardiac myocytes. He decides to obtain electrocardiograms on patients with this disease in order to see how the wave patterns and durations change over time. While studying these records, he asks a medical student who is working with him to interpret the traces. Specifically, he asks her to identify the part that represents initial ventricular depolarization. Which of the following characteristics is most consistent with this feature of the electrocardiogram?

Normal duration defined as less than 120 milliseconds

Elevated in patients with full thickness ischemic injury of the heart

Becomes peaked in states of hyperkalemia

Becomes prominent in states of hypokalemia

Normal duration defined as less than 200 milliseconds

A 34-year-old woman presents to her primary care provider after intermittently passing bright pink urine over several days. She is concerned this discoloration is due to blood. Her medical history is unremarkable, she denies being sick in the past weeks and has only taken a couple of diclofenac capsules for pelvic pain associated to her menstrual period. She does not drink alcohol or smoke cigarettes. At the doctor’s office, her blood pressure is 150/90 mm Hg, pulse is 80/min, respiratory rate is 18/min, and temperature is 36.5°C (97.7°F). On physical exam, she has 2+ pitting edema up to her knees. A urinalysis is taken which shows red blood cells, red blood cell casts, and acanthocytes. No proteinuria was detected. Her serum creatinine is 2.4 mg/dL, blood urea nitrogen 42 mg/dL, serum potassium 4.8 mEq/L, serum sodium 140 mEq/L, serum chloride 102 mEq/L. Which of the following is the most appropriate next step in the management of this case?

Intravenous fluid therapy and electrolyte correction

Noncontrast computed tomography

Principles of cross-sectional imaging for USMLE Step 3: High-Yield Anatomy Lessons

Imaging Planes - Slicing & Dicing

Axial (Transverse): Horizontal slices dividing the body into superior (top) and inferior (bottom) portions. Imagine looking up from the patient's feet.
Coronal (Frontal): Vertical slices dividing the body into anterior (front) and posterior (back) portions.
Sagittal: Vertical slices dividing the body into left and right portions.
- Midsagittal: A slice directly down the midline.
- Parasagittal: Any slice parallel to the midline.

Anatomical Planes: Sagittal, Coronal, and Transverse

⭐ Most CT scans are acquired in the axial plane; software then reconstructs coronal and sagittal views (Multiplanar Reconstruction or MPR).

CT Principles - Donuts of Density

CT measures tissue radiodensity in Hounsfield Units (HU), creating a grayscale map.
Density Scale (HU):
- Air: -1000 (black)
- Fat: -100 to -50
- Water: 0
- Soft Tissue: +20 to +100
- Bone: +1000 (white)
Terminology:
- Hyperdense (bright): Bone, acute blood, contrast.
- Hypodense (dark): Air, fat, edema.
- Isodense: Similar density to reference tissue.

⭐ Intravenous contrast opacifies blood vessels and vascular organs, increasing their HU value and making them appear brighter, which is crucial for identifying pathology.

MRI Principles - Magnetic Personality

Core Principle: Exploits the magnetic properties of hydrogen (¹H) protons. A strong external magnetic field ($B_0$) aligns these protons.
Signal Generation:
- A radiofrequency (RF) pulse ($B_1$) temporarily knocks protons out of alignment.
- As protons "relax" back to equilibrium, they emit a detectable RF signal.
Relaxation & Tissue Contrast:
- T1-weighted: Measures longitudinal relaxation. Excellent for anatomy.
  - Fat is bright; water is dark.
- T2-weighted: Measures transverse relaxation. Best for pathology (edema).
  - Water/fluid and fat are bright.

⭐ Gadolinium-based contrast agents shorten the T1 relaxation time of tissues, causing them to appear much brighter on T1-weighted images.

Radiocontrast & Safety - Glow & Go Carefully

Iodinated Contrast (CT/Angio):
- ⚠️ Risk: Contrast-Induced Nephropathy (CIN), especially with pre-existing renal disease (↑Cr).
- Prevention: Pre- and post-procedure IV hydration (isotonic saline). Check eGFR/Cr. Hold metformin.
- Allergy: Low risk of true IgE-mediated allergy. Pre-medicate with corticosteroids for prior moderate/severe reactions.
Gadolinium-based Contrast (MRI):
- ⚠️ Risk: Nephrogenic Systemic Fibrosis (NSF) in patients with severe renal dysfunction.
- Contraindication: Avoid in patients with eGFR < 30 mL/min/1.73m².

⭐ The most critical preventive measure for CIN is aggressive periprocedural IV hydration with normal saline.

Pathophysiology of Contrast-Induced Nephropathy

High‑Yield Points - ⚡ Biggest Takeaways

CT utilizes X-rays; bone appears white (hyperdense) and air appears black (hypodense). It is the primary modality for evaluating acute hemorrhage and bony structures.

MRI offers superior soft-tissue contrast without using ionizing radiation, making it ideal for assessing the brain, spine, and joints.

On T1-weighted MRI, fat is bright; on T2-weighted MRI, both fat and water (pathology) are bright.

Ultrasound is a real-time, non-invasive imaging technique, excellent for gallbladder, obstetrics, and vascular applications.

Always identify the imaging plane: axial (cross-section), coronal (front/back), and sagittal (left/right).

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