**Core Concept**
Cardiac polyp refers to a type of polypoid thrombus that forms on the endocardium, particularly on the ventricular surface of the heart. It is a post-mortem finding, not a clinical entity.

**Why the Correct Answer is Right**
Cardiac polyp is a type of thrombus that forms after death, usually as a result of the stasis of blood within the heart chambers. This can occur due to the cessation of circulation, leading to the coagulation of blood on the endocardial surface. The cardiac polyp is a characteristic finding in post-mortem examinations, and its presence can be used to differentiate between ante-mortem and post-mortem events.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute nephritis is a renal disorder characterized by inflammation of the kidneys, which is not directly related to the formation of cardiac polyps.

**Option B:** Endocarditis is an infection of the inner lining of the heart, which can lead to the formation of vegetations, not polyps.

**Option C:** Septicaemia is a systemic infection that can lead to the formation of septic emboli, not cardiac polyps.

**Clinical Pearl / High-Yield Fact**
When examining a post-mortem specimen, the presence of a cardiac polyp can indicate that the death was not due to a cardiac cause, but rather due to other underlying conditions.

**✓ Correct Answer: D. Postmoem clot**