**Core Concept**
Stress fractures are a type of overuse injury that occurs when repetitive stress on a bone exceeds its ability to repair itself. This can lead to microfractures, which are cracks in the bone. The earliest changes in stress fractures are detectable through imaging and clinical evaluation.

**Why the Correct Answer is Right**
The earliest changes in stress fractures are seen on magnetic resonance imaging (MRI) as bone marrow edema, which is inflammation of the bone marrow. This is due to the bone's response to the repetitive stress, leading to an increase in blood flow and fluid accumulation in the bone marrow. The bone edema is a sensitive indicator of early stress fracture, even before the formation of a visible fracture line.

**Why Each Wrong Option is Incorrect**
**Option A:** X-rays are not sensitive enough to detect early changes in stress fractures, as they require a significant amount of bone damage to show a visible fracture line.
**Option B:** Laboratory tests, such as serum alkaline phosphatase levels, may be elevated in stress fractures but are not specific or sensitive enough for early detection.
**Option C:** Clinical symptoms, such as pain and swelling, may be present in stress fractures but are not specific or sensitive enough for early detection.

**Clinical Pearl / High-Yield Fact**
Bone marrow edema is a sensitive indicator of early stress fracture, and MRI is the imaging modality of choice for detecting this condition.

**Correct Answer:** B. Bone marrow edema on MRI.