**Core Concept**
Imipenem is a carbapenem antibiotic that has a broad spectrum of activity against both Gram-positive and Gram-negative bacteria. However, it is unstable in the presence of renal dehydropeptidase, an enzyme found in the renal tubules, which can lead to its metabolism and reduced efficacy.

**Why the Correct Answer is Right**
Cilastatin is a dehydropeptidase inhibitor that is coadministered with imipenem to prevent its degradation by renal dehydropeptidase. By inhibiting this enzyme, cilastatin allows imipenem to be excreted by the kidneys in its active form, thereby maintaining its efficacy. This combination is essential to ensure that imipenem reaches therapeutic concentrations in the bloodstream and tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cilastatin does not act as a beta-lactamase inhibitor, which is a different mechanism of action that is not relevant to imipenem's stability.
**Option B:** This option is incorrect because cilastatin does not enhance the absorption of imipenem; rather, it prevents its degradation in the kidneys.
**Option C:** This option is incorrect because cilastatin does not have a direct effect on the renal excretion of imipenem; it simply prevents its metabolism by renal dehydropeptidase.

**Clinical Pearl / High-Yield Fact**
When administering imipenem-cilastatin, it is essential to remember that the combination is stable in acidic environments, which allows it to be administered intravenously without significant degradation.

**Correct Answer:** D. Cilastatin inhibits renal dehydropeptidase, allowing imipenem to be excreted by the kidneys in its active form.