Step 1: The clinical picture of severe ear pain of sudden onset with blood-filled blisters on the drum identifies bullous myringitis.
Step 2: The organism classically linked with this condition is Mycoplasma pneumoniae, with viral agents like influenza as another association.
Step 3: Examination shows serosanguinous haemorrhagic bullae over the tympanic membrane and deep external canal, often with mild conductive hearing loss.
Step 4: After resolution the healed drum shows the typical Sago-grain look. Bacteria such as Pseudomonas or Pneumococcus and the fungus Candida do not cause this entity.
\[\boxed{\text{Mycoplasma}}\]