What is the approach to differential diagnosis for various conditions in the MRCP (Membership of the Royal Colleges of Physicians) PACES (Practical Assessment of Clinical Examination Skills) exam?

MRCP PACES Differential Diagnosis Framework

The MRCP PACES examination requires systematic differential diagnosis generation across all clinical stations, prioritizing pattern recognition through aggregation of clinical findings, selection of key pivotal features, and structured pruning of diagnostic possibilities based on likelihood and clinical context. 1

Core Diagnostic Approach

Station-Specific Differential Frameworks

Respiratory System

• Chronic dyspnea with wheeze: Asthma (assess symptom frequency, medication compliance, exacerbating factors), COPD, cardiac asthma, dysfunctional breathing patterns (particularly with anxiety), bronchiectasis 2
• Acute breathlessness with hypoxia: Pulmonary embolism (if normal chest radiograph increases likelihood), pneumonia, pneumothorax, acute heart failure, pulmonary edema 3
• Pleural effusion: Heart failure, malignancy, infection (parapneumonic/empyema), pulmonary embolism, hypoalbuminemia 3

Cardiovascular System

• Heart failure presentation (progressive dyspnea, peripheral edema, bilateral crackles): Left ventricular systolic dysfunction, diastolic dysfunction, right heart failure, valvular disease (particularly mitral regurgitation or aortic stenosis), restrictive cardiomyopathy 4

  • Key pivotal finding: S3 gallop (87-92% specific for left ventricular dysfunction) 4
  • S4 is less specific (72-80%) and suggests hypertension or aortic stenosis 4

• Chest pain in young patients: Musculoskeletal (most common), gastroesophageal reflux, pulmonary causes, cardiac etiologies (coronary anomalies, myocarditis, pericarditis, hypertrophic cardiomyopathy) 3

• Syncope: Vasovagal (neural-mediated), cardiac arrhythmia (long QT, conduction abnormalities), structural heart disease, orthostatic hypotension, neurological causes 3

Abdominal System

• Chronic cholestasis (elevated ALP + GGT ± bilirubin):

  1. If dilated ducts on ultrasound: Choledocholithiasis, malignancy (pancreatic head, cholangiocarcinoma), stricture 3
  2. If normal ducts: Primary biliary cholangitis (check AMA, ANA-sp100/gp210), primary sclerosing cholangitis (MRCP shows multifocal strictures/dilatations), secondary sclerosing cholangitis, small duct PSC 3
  3. If MRCP negative: Parenchymal disease (liver biopsy), genetic causes (ABCB4 deficiency) 3

• Liver mass in non-cirrhotic liver: Intrahepatic cholangiocarcinoma (requires CK7+, CK20-, CDX2/SATB2-, TTF1-), hepatocellular carcinoma (unusual without cirrhosis), liver metastases (check CK7, CK20, CDX2/SATB2, TTF1, GATA3 in women), benign lesions (hemangioma, focal nodular hyperplasia, adenoma) 3

Neurological System

• Infant with motor delay/abnormal tone (age <5 months corrected age):

  • Cerebral palsy (use General Movements assessment + MRI) 3
  • Genetic/metabolic disorders
  • Neuromuscular disorders
  • Spinal cord pathology
  • Key approach: Combined assessment using HINE (Hammersmith Infant Neurological Examination), General Movements, and MRI provides highest accuracy 3

• Progressive weakness: Motor neuron disease, myopathy, neuropathy, neuromuscular junction disorders, spinal cord compression

Endocrine/Metabolic

• Thyroid enlargement: Graves' disease, multinodular goiter, thyroiditis, malignancy
• Cushingoid features: Exogenous steroids (most common), pituitary adenoma, adrenal adenoma/carcinoma, ectopic ACTH

Critical Diagnostic Principles

Pattern Recognition Strategy

1. Aggregate findings into recognizable patterns rather than analyzing isolated symptoms 1
2. Identify the pivotal finding - the single most discriminating feature (e.g., S3 for heart failure, multifocal strictures on MRCP for PSC) 1, 3, 4
3. Generate comprehensive cause list including common and uncommon diseases 5
4. Prune systematically based on supporting/contradicting features 1

Diagnostic Test Sequencing

• History provides 76% of diagnoses - most critical component 6
• Physical examination adds 12% and increases diagnostic confidence 6
• Investigations confirm 11% but primarily exclude alternatives and increase confidence from 7.1/10 to 9.3/10 6

Station-Specific Pitfalls

Respiratory Station:

• Never attribute all dyspnea to anxiety without excluding organic causes, especially in elderly patients 2
• Normal chest radiograph in acute breathless hypoxic patient increases PE likelihood 3

Cardiovascular Station:

• Fixed split S2 indicates atrial septal defect, not heart failure 4
• Systolic click suggests mitral valve prolapse, not heart failure 4

Abdominal Station:

• PSC diagnosis requires high-quality MRCP (visualizes third-order ducts without artifacts) showing multifocal strictures/dilatations AND exclusion of secondary causes 3
• 50-80% of PSC patients have IBD - always investigate 3
• Liver biopsy not mandatory if cholangiography diagnostic for PSC 3
• For suspected cholangiocarcinoma, immunohistochemistry panel (CK7, CK20, CDX2/SATB2, TTF1, GATA3) differentiates from metastases 3

Neurological Station:

• For infants <5 months with suspected cerebral palsy: combine General Movements assessment + MRI + clinical history for highest accuracy 3
• High-risk designation appropriate when diagnosis uncertain but suspicion high 3

Key Examination Maneuvers by System

• Cardiovascular: Assess for S3 (early diastolic, low-pitched), jugular venous distension, hepatojugular reflux 4
• Respiratory: Document oxygen saturation, respiratory rate (tachypnea >90% in acute heart failure) 4
• Neurological: Orthostatic vital signs for syncope, detailed motor examination with HINE scoring for infants 3