Achalasia: loss of inhibitory (NO/VIP-secreting) neurons in Auerbach's plexus → unopposed LES tone + aperistalsis. Gold-standard diagnosis is high-resolution manometry. Barium swallow shows the classic bird-beak; endoscopy is mandatory to exclude pseudoachalasia (carcinoma of GEJ mimicking achalasia — always suspect if age >60, rapid weight loss, or short history).
Modified Johnson's classification (5 types). NEET trap: Type I (incisura, normal/low acid) is the MC gastric ulcer — do NOT confuse with duodenal ulcer (high acid). Type IV near GEJ = highest malignancy risk, technically most difficult surgery. Type V = NSAID-induced, unique in having no H. pylori link.
Dumping syndrome occurs after any gastric surgery that disrupts pyloric function (gastrectomy, pyloroplasty, vagotomy). Early and late have completely different mechanisms — the exam frequently tests this distinction. Key identifier: early = vasomotor (flushing, diarrhoea immediately after eating); late = neuroglycopenic (sweating, confusion hours later).