**Core Concept**
Focal change in a renal biopsy specimen refers to the presence of abnormal changes that are limited to a specific area or sector of the kidney, as opposed to diffuse changes that affect the entire organ. This concept is crucial in histopathological diagnosis, as it helps differentiate between focal and diffuse glomerular diseases.

**Why the Correct Answer is Right**
The pathologist identifies focal change when the abnormal changes are confined to less than 50% of the glomeruli examined in the biopsy sample. This indicates that the disease process is localized and may not be representative of the entire kidney. The pathologist may use various criteria, such as the number of glomeruli affected, the severity of the changes, and the presence of other abnormalities, to determine the extent of the focal change.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a percentage or a criterion for determining focal change. Focal change cannot be diagnosed solely based on the presence of abnormal changes without considering their extent.
**Option B:** This option is incorrect because it does not provide a clear criterion for determining focal change. A percentage or a specific number of glomeruli affected is necessary to diagnose focal change.

**Clinical Pearl / High-Yield Fact**
When interpreting renal biopsy specimens, pathologists use the term "focal" to describe changes that are limited to less than 50% of the glomeruli examined, in contrast to "diffuse" changes that affect the entire kidney.

**Correct Answer: C.**