**Core Concept**
Körner's septum is a fibrous structure that separates the left atrium from the left ventricle in the heart. It is a remnant of the embryological development of the heart, specifically the division of the atrioventricular canal.

**Why the Correct Answer is Right**
The development of the heart involves the formation of the atrioventricular canal, which eventually separates into the mitral and tricuspid valves. However, in some cases, a fibrous remnant of this canal, known as Körner's septum, persists and can be found in the left atrium. This structure is usually small and insignificant but can be a useful landmark for anatomists and pathologists.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a location or structure related to Körner's septum.
**Option B:** This option is incorrect because it refers to a different anatomical structure, the moderator band, which is found in the right ventricle.
**Option C:** This option is incorrect because it refers to a different anatomical structure, the crista terminalis, which is found in the right atrium.

**Clinical Pearl / High-Yield Fact**
Körner's septum is a small, fibrous remnant of the atrioventricular canal that can be found in the left atrium. It is a useful landmark for anatomists and pathologists, but its clinical significance is limited.

**Correct Answer: D. Left atrium. Körner's septum is a fibrous structure found in the left atrium.**