**Core Concept**
PET (Positron Emission Tomography) scan is a diagnostic imaging technique that uses a radioactive tracer to visualize metabolic processes in the body. The tracer is a molecule that is labeled with a positron-emitting isotope, which emits a positron that annihilates with an electron, producing gamma rays detectable by the PET scanner.

**Why the Correct Answer is Right**
The correct answer is likely Fluorodeoxyglucose (FDG), a glucose analog that is taken up by cells in proportion to their metabolic rate. FDG is phosphorylated by hexokinase and trapped within the cell, allowing the PET scanner to detect the localized accumulation of the radioactive tracer. This makes FDG an ideal tracer for detecting areas of high metabolic activity, such as tumors or areas of inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose - While glucose is the target molecule for FDG, glucose itself is not typically used in PET scans due to its short half-life and rapid metabolism.

**Option B:** Thallium-201 - Thallium-201 is a radioactive isotope used in some types of nuclear medicine imaging, but it is not commonly used in PET scans.

**Option C:** Technetium-99m - Technetium-99m is a radioactive isotope used in a wide range of nuclear medicine imaging applications, but it is not typically used in PET scans.

**Clinical Pearl / High-Yield Fact**
FDG-PET scans are particularly useful for detecting and staging cancer, as well as monitoring the response of tumors to treatment. However, FDG can accumulate in areas of inflammation, such as in infections or inflammatory bowel disease, which can lead to false-positive results.

**Correct Answer:** D. Fluorodeoxyglucose (FDG)