**Core Concept**
Constrictive pericarditis and restrictive cardiomyopathy are two conditions that can present with similar symptoms, making differentiation crucial. The key lies in understanding the pathophysiology of both conditions and identifying distinctive features.

**Why the Correct Answer is Right**
Constrictive pericarditis is characterized by the scarring and thickening of the pericardium, leading to a rigid and non-compliant pericardial sac. This results in a uniform elevation of diastolic pressures in all cardiac chambers, as seen in option **a)** Diastolic pressures are equalised. Additionally, a thickened pericardium is a hallmark of constrictive pericarditis, as mentioned in option **c)** Thick pericardium is present. This thickening can also lead to a mild pericardial effusion, as stated in option **b)** There is mild pericardial effusion. However, restrictive cardiomyopathy is characterized by an increase in right ventricular (RV) size, making option **d)** RV size is increased less likely.

**Why Each Wrong Option is Incorrect**
**Option B:** c is incorrect because while a thickened pericardium is present in constrictive pericarditis, it is not a distinguishing feature from restrictive cardiomyopathy, where the pericardium may also be thickened.
**Option C:** ac is incorrect because while diastolic pressures are equalised in constrictive pericarditis, a thick pericardium is not a distinguishing feature from restrictive cardiomyopathy.
**Option D:** ab is incorrect because while diastolic pressures are equalised and there is a mild pericardial effusion in constrictive pericarditis, an increased RV size is more characteristic of restrictive cardiomyopathy.

**Clinical Pearl / High-Yield Fact**
When differentiating between constrictive pericarditis and restrictive cardiomyopathy, remember that the presence of a thickened pericardium and uniform elevation of diastolic pressures are key features of constrictive pericarditis.

**✓ Correct Answer: A. ad. Diastolic pressures are equalised and Thick pericardium is present are the features favouring constrictive pericarditis.**