A 54-year-old man comes to the physician for the evaluation of difficulty swallowing of both solids and liquids for 1 month. During the past 5 months, he has also had increased weakness of his hands and legs. He sails regularly and is unable to hold the ropes as tightly as before. Ten years ago, he was involved in a motor vehicle collision. Examination shows atrophy of the tongue. Muscle strength is decreased in the right upper and lower extremities. There is muscle stiffness in the left lower extremity. Deep tendon reflexes are 1+ in the right upper and lower extremities, 3+ in the left upper extremity, and 4+ in the left lower extremity. Plantar reflex shows an extensor response on the left foot. Sensation to light touch, pinprick, and vibration is intact. Which of the following is the most likely diagnosis?

Subacute combined degeneration of spinal cord

Cervical spondylosis with myelopathy

A 74-year-old woman is brought by ambulance to the emergency department and presents with a complaint of excruciating chest pain that started about 45 minutes ago. The patient was sitting in the garden when she 1st noticed the pain in the upper abdomen. The pain has persisted and now localizes underneath of the sternum and the left shoulder. Milk of magnesia and aspirin were tried with no relief. The patient had previous episodes of chest pain that were of lesser intensity and rarely lasted more than 10 minutes. She is diabetic and has been managed for hypertension and rheumatoid arthritis in the past. On examination, the patient is breathless and sweating profusely. The vital signs include blood pressure 140/90 mm Hg and heart rate 118/min. The electrocardiogram (ECG) shows Q waves in leads V2 and V3 and raised ST segments in leads V2, V3, V4, and V5. Laboratory studies (including cardiac enzymes at 6 hours after admission show: Hematocrit 45% Troponin T 1.5 ng/mL Troponin I 0.28 ng/mL Creatine kinase (CK)-MB 0.25 ng/mL The patient is admitted and started on analgesia and reperfusion therapy. She shows initial signs of recovery until the 6th day of hospitalization when she starts vomiting and complaining of dizziness. Physical examination findings at this time included heart rate 110/min, temperature 37.7°C (99.9°F), blood pressure 90/60 mm Hg. Jugular venous pressure is 8 cm. A harsh pansystolic murmur is present at the left lower sternal border. ECG shows sinus tachycardia and ST-segment elevation with terminal negative T waves. Laboratory studies show: Hematocrit 38% Troponin T 1.15ng/mL Troponin I 0.18 ng/mL CK-MB 0.10 ng/mL Which of the following best explains the patient's current clinical condition?

Acute ventricular septal rupture complicating myocardial infarction

Aortic dissection complicating myocardial infarction

A new myocardial infarction (re-infarction)

Acute pericarditis complicating myocardial infarction

Cardiac tamponade complicating myocardial infarction

A 48-year-old woman presents to the emergency department because of increasingly severe right upper abdominal pain, fever, and non-bloody vomiting for the last 5 hours. The pain is dull, intermittent, and radiates to her right shoulder. During the past 3 months, she has had recurring abdominal discomfort after meals. The patient underwent an appendectomy more than 30 years ago. She has hypertension, diabetes mellitus type 2, and chronic back pain. She takes bisoprolol, metformin, and ibuprofen daily. She is 171 cm (5 ft 6 in) tall and weighs 99 kg (218 lb). Her BMI is 35.2 kg/m2. She appears uncomfortable and is clutching her abdomen. Her temperature is 38.5°C (101.3°F), pulse is 108/min, and blood pressure is 150/82 mm Hg. Abdominal examination shows right upper quadrant abdominal tenderness and guarding. Upon deep palpation of the right upper quadrant, the patient pauses during inspiration. Laboratory studies show the following: Blood Hemoglobin 13.1 g/dL Leukocyte count 10,900/mm3 Platelet count 236,000/mm3 Mean corpuscular volume 89/µm3 Serum Urea nitrogen 28 mg/dL Glucose 89 mg/dL Creatinine 0.7 mg/dL Bilirubin Total 1.6 mg/dL Direct 1.1 mg/dL Alkaline phosphatase 79 U/L Alanine aminotransferase (ALT, GPT) 28 U/L Aspartate aminotransferase (AST, GOT) 32 U/L An X-ray of the abdomen shows no abnormalities. Further evaluation of the patient is most likely to reveal which of the following?

History of multiple past pregnancies

History of recent travel to Indonesia

Frequent, high-pitched bowel sounds on auscultation

Elevated carbohydrate-deficient transferrin

History of recurrent sexually transmitted infections

A previously healthy 33-year-old woman comes to the physician because of pain and sometimes numbness in her right thigh for the past 2 months. She reports that her symptoms are worse when walking or standing and are better while sitting. Three months ago, she started going to a fitness class a couple times a week. She is 163 cm (5 ft 4 in) tall and weighs 88 kg (194 lb); BMI is 33.1 kg/m2. Her vital signs are within normal limits. Examination of the skin shows no abnormalities. Sensation to light touch is decreased over the lateral aspect of the right anterior thigh. Muscle strength is normal. Tapping the right inguinal ligament leads to increased numbness of the affected thigh. The straight leg test is negative. Which of the following is the most appropriate next step in management of this patient?

Advise patient to wear looser pants

Blood work for inflammatory markers

Anatomical approach to differential diagnosis for USMLE Step 1: High-Yield Diagnosis Lessons

Anatomical DDx - Location, Location, Location!

Principle: Pinpoint the symptom's location to identify underlying anatomical structures. This forms the basis of your initial differential diagnosis (DDx).
Example: Right Lower Quadrant (RLQ) pain points to the appendix, cecum, terminal ileum, or ovary/ureter.

Abdominal Quadrants and Underlying Organs

⭐ Don't forget referred pain! Kehr's sign (spleen rupture causing left shoulder pain via C3-C5 irritation) is a classic example where the pain location is distant from the pathology.

Torso Troubles - Chest & Abdominal Maps

Anatomical location is the first branch point in narrowing differentials for chest and abdominal pain. Think location, then organ, then pathology.

Chest Pain Localization
- Retrosternal: MI, GERD, Esophageal spasm, Aortic dissection
- Pleuritic (sharp, worse with inspiration): PE, Pneumonia, Pleurisy, Pericarditis
- Chest Wall (tender to palpation): Costochondritis, Rib fracture, Herpes zoster
Abdominal Pain Localization
- Use the 4-quadrant system as a primary map.
- 📌 Mnemonic (Pancreatitis): GET SMASHED for causes.

Abdominal Pain: Anatomical Differential Diagnosis

⭐ Kehr's Sign: Spleen irritation (e.g., rupture from trauma) can present as referred pain in the left shoulder tip. This is due to irritation of the diaphragm and phrenic nerve (C3-C5).

Head & Limbs - Neurological & MSK Clues

Anatomical Sieve: Localize the lesion first. Is it Central (UMN) or Peripheral (LMN)?

Head/Face Clues:
- Cranial Nerve Palsies: Diplopia (CN III, IV, VI), facial droop (CN VII), dysarthria/dysphagia (CN IX, X, XII).
- Jaw Deviation: Toward side of lesion (CN V motor).
Limb Clues:
- Upper vs. Lower Motor Neuron (UMN/LMN) Signs:

Feature	UMN Lesion	LMN Lesion
Tone	↑ Spasticity	↓ Flaccidity
Reflexes	↑ Hyperreflexia	↓ Hyporeflexia
Atrophy	Disuse (late)	Denervation (severe)
Babinski	Present	Absent

⭐ Winging of the Scapula: Injury to the Long Thoracic Nerve (C5, C6, C7 roots) paralyzes the serratus anterior muscle. Often iatrogenic (axillary surgery).

High‑Yield Points - ⚡ Biggest Takeaways

The anatomical approach first localizes the lesion, then builds a differential diagnosis based on the structures involved.

This method is essential for neurological deficits, where precise localization (e.g., cortex, brainstem) is critical.

For any localized finding, apply a structured mnemonic like VINDICATE.

In acute presentations, always prioritize life-threatening vascular events like ischemia or hemorrhage.

Mass lesions on imaging should always raise suspicion for neoplasm, abscess, or hematoma.

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