**Core Concept:**
The question is asking about the typical appearance of the coronary artery cusps in an adult heart. In a healthy heart, there are typically three cusps present: the left coronary cusp (LCC), the right coronary cusp (RCC), and the non-coronary cusp (NCC). The fourth and fifth cusps are rarely seen in adult hearts.

**Why the Correct Answer is Right:**
The correct answer is D (rarely seen), as the fourth and fifth cusps are exceptions and not the norm in adult hearts. In children and fetuses, these additional cusps can be present, but in adults, they are usually absent or replaced by the fusion of the adjacent cusps.

**Why Each Wrong Option is Incorrect:**
A (left coronary cusp) is incorrect because the left coronary cusp is actually present in the majority of adult hearts.
B (right coronary cusp) is also incorrect as the right coronary cusp is present in most adult hearts.
C (non-coronary cusp) is incorrect because the non-coronary cusp is the same as the fourth cusp, which we've already established is rarely seen in adults.

**Clinical Pearl / High-Yield Fact:**
In the context of coronary artery anatomy, it is essential to understand the normal cusp arrangement in adults. These additional cusps, if present, could be indicative of congenital heart defects, such as tetralogy of Fallot or tricuspid atresia, particularly in neonates and infants. Knowledge of these exceptions helps in differentiating between normal and abnormal appearances in various cardiac imaging modalities like echocardiography.

**Correct Answer:** D (rarely seen)