**Core Concept**
PDS (Phenytoin) is an antiepileptic medication with a narrow therapeutic index, requiring careful monitoring of plasma levels to avoid toxicity. Its absorption characteristics are critical in clinical practice, especially in patients with gastrointestinal disorders.

**Why the Correct Answer is Right**
PDS is absorbed within the duodenum and upper small intestine, primarily through passive diffusion. The absorption rate of PDS is slow and variable due to its high lipophilicity, which allows it to dissolve in the lipid-rich bile salts in the duodenum. This results in a biphasic absorption profile, with an initial rapid phase followed by a slower, more sustained phase.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PDS is not primarily absorbed in the stomach, where pH and enzymatic degradation can significantly affect absorption.
**Option B:** This option is incorrect because while PDS can be absorbed in the small intestine, it is not the primary site of absorption.
**Option C:** This option is incorrect because PDS is not significantly absorbed in the large intestine, where water and electrolyte absorption predominate.

**Clinical Pearl / High-Yield Fact**
When administering PDS orally, it is essential to ensure the patient is fasted for at least 4-6 hours to minimize first-pass metabolism and enhance absorption.

**Correct Answer: D. The duodenum and upper small intestine.**