Assertion (A) is true — otitis, especially otitis externa or otitis media, causes severe ear pain.
This pain occurs because the skin lining of the ear canal and tympanic membrane is tightly attached to underlying cartilage and bone.
Reason (R) is also true — the tight adherence of skin to cartilage limits space for swelling, causing intense pain.
Additionally, congestion worsens when lying down due to increased vascular pooling.
This mechanism directly explains why pain is severe in otitis.
Therefore, both statements are true and (R) is the correct explanation of (A).