**Core Concept**
Intestinal alkaline phosphatase (IAP) is a zinc-dependent enzyme involved in the hydrolysis of phosphate esters, playing a crucial role in the digestion and absorption of nutrients in the small intestine. Non-competitive inhibitors bind to a site other than the active site of the enzyme, altering its conformation and reducing its activity.

**Why the Correct Answer is Right**
L-cysteine is a non-competitive inhibitor of IAP. It binds to a site near the active site, causing a conformational change in the enzyme that reduces its ability to hydrolyze phosphate esters. This inhibition is reversible and can be overcome by increasing the concentration of the substrate. The mechanism of inhibition involves the formation of a disulfide bond between the cysteine residue of the enzyme and the sulfhydryl group of L-cysteine.

**Why Each Wrong Option is Incorrect**
**Option A:** Does not inhibit IAP.
**Option B:** Actually a competitive inhibitor of IAP, not a non-competitive one.
**Option C:** Does not have any known effect on IAP activity.

**Clinical Pearl / High-Yield Fact**
Non-competitive inhibition is a common mechanism of enzyme regulation in the body, and understanding these mechanisms is crucial for understanding various disease states and developing effective treatments.

**Correct Answer: C. L-cysteine**