IM Shelf Diagnostic Drill

Interactive Zoomable High-Yield Diagnostic Algorithms (Click Any Diagram to Zoom)

1. The 4 Golden Rules of NBME Diagnostic Questions

1. Decode the Question Stem Wording:
   • "Best initial test" / "First step in evaluation": Pick the least invasive, most accessible screening test (e.g., ECG for chest pain, chest X-ray for dyspnea, RUQ ultrasound for biliary colic, Ankle-Brachial Index for claudication).
   • "Most accurate test" / "Most specific test" / "Definitive diagnosis": Pick the gold standard confirmatory test, even if it is invasive (e.g., coronary angiography, right heart catheterization, tissue biopsy, ERCP).
   • "Next best step in management": Be careful—this encompasses both diagnostic tests and treatments. If the diagnosis is already clinically certain or the patient is unstable, the answer is a treatment, not another test.
2. The Hemodynamic Stability Rule ("Donut of Death"):
   • Never send a hemodynamically unstable patient (hypotension, tachycardia, altered mental status) to the CT or MRI scanner.
   • Choose a bedside test (e.g., portable chest X-ray, bedside FAST ultrasound, transthoracic or transesophageal echo) or immediate stabilization/intervention (e.g., synchronized cardioversion, needle decompression, emergent endoscopy after airway protection).
3. The 5 "Treat First, Confirm Second" Exceptions:
   • Giant Cell (Temporal) Arteritis (headache, jaw claudication, visual loss, age greater than 50) → Give high-dose corticosteroids immediately to prevent permanent blindness, then order a temporal artery biopsy.
   • Malignant Spinal Cord Compression (back pain, motor weakness, sensory level, hyperreflexia in a cancer patient) → Give IV dexamethasone immediately, then order an emergent spine MRI.
   • Tension Pneumothorax / Cardiac Tamponade with Shock → Perform needle decompression / pericardiocentesis immediately based on clinical findings; do not delay for a chest X-ray.
   • Acute Bacterial Meningitis when LP is delayed → Obtain blood cultures and start empiric IV antibiotics + dexamethasone before sending the patient for a head CT.
     Mnemonic for needing a Head CT before LP: FAINTS — Focal neurologic deficit, Altered mental status, Immunocompromised state, New-onset seizure, Tigns of papilledema/increased ICP, Sixty years of age or older.
   • High-Probability Pulmonary Embolism (high Wells score, no bleeding risk) → Start empiric anticoagulation (heparin) first, then order CT pulmonary angiography.
4. The Contrast & Kidney Filter:
   • Before choosing any CT with IV contrast (CT angiogram of the chest/abdomen), check the vignette for creatinine / eGFR and contrast allergies.
   • If eGFR is less than 30 mL/min or the patient has a severe contrast allergy:
     • Suspected PE → Order a V/Q (ventilation-perfusion) scan (if chest X-ray is normal) or lower-extremity compression ultrasound.
     • Suspected Aortic Dissection → Order a Transesophageal Echocardiogram (TEE) (or MR angiography if stable).

2. The Imaging Modality Rulebook: How to Pick the Right Scan

3. System-by-System Diagnostic Cheat Sheets

Cardiology & Vascular

Pulmonology

Gastroenterology & Hepatobiliary

Nephrology & Endocrinology

Neurology, Rheumatology & Infectious Disease

4. Quick Mnemonic for "Biopsy vs. Imaging vs. Function Test"

When stuck between two diagnostic answers on exam day, use "S-F-T" (Screen → Function/Fluid → Tissue):

1. Screen (Anatomy/Bedside): Start with ECG, X-ray, or Ultrasound to rule out emergencies or obvious structural problems.
2. Function / Fluid (Biochemistry before Advanced Imaging): In Endocrinology and Effusions/Joints, always prove the biochemical or fluid abnormality first (TSH, metanephrines, cortisol suppression, arthrocentesis, thoracentesis) before ordering a CT or MRI. (Finding an incidentaloma on CT without labs first is a classic NBME trap!)
3. Tissue / Tube (Definitive): If the question asks for the most accurate or confirmatory test for cancer, vasculitis, sarcoidosis, or GI mucosa, the answer is almost always endoscopy/biopsy or angiography/catheterization.