**Core Concept:**
During haemodialysis, the patient experiences a sudden drop in blood volume due to ultrafiltration, which can lead to hypotension and subsequent reflex sympathetic activation. This can manifest as chest pain and back pain due to increased cardiac workload and vasoconstriction.

**Why the Correct Answer is Right:**
In this scenario, the patient is experiencing post-dialysis hypotension, which triggers a reflex sympathetic response. This reflex causes vasoconstriction, resulting in chest and back pain. The pain resolves over time as the patient's blood pressure and volume stabilize.

**Why Each Wrong Option is Incorrect:**
A. This option does not address the mechanism of pain in post-dialysis patients.
B. Although pain might be present, this option does not explain the cause specifically, focusing on hypotension instead.
C. This option is unrelated to the symptoms described in the question.
D. Although this option mentions pain after dialysis, it does not explain the underlying mechanism, focusing on hypotension instead.

**Clinical Pearl:**
Haemodialysis-induced pain is a common phenomenon and should be expected by patients. Instructing patients to remain calm and reassured can help them cope with these symptoms, which typically resolve on their own within a few minutes as the body adapts to the changes during dialysis.

**Correct Answer:** D.