**Core Concept**
The mechanism of action of spermicides involves the disruption of the cell membrane of sperm, ultimately leading to their immobilization and death. Spermicides contain nonoxynol-9, a surfactant that alters the lipid bilayer of the sperm cell membrane.

**Why the Correct Answer is Right**
Nonoxynol-9 works by denaturing the proteins on the surface of the sperm cell membrane, causing the membrane to become permeable and leading to the loss of intracellular ions and water. This results in the sperm becoming immobile and unable to fertilize the egg. The surfactant properties of nonoxynol-9 also help to break down the acrosome reaction, a critical step in sperm penetration of the egg.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because spermicides do not work by causing an allergic reaction or an immune response.
**Option B:** This option is incorrect because spermicides do not work by altering the pH of the vagina or causing a chemical reaction.
**Option C:** This option is incorrect because spermicides do not work by physically blocking the cervix or uterus.

**Clinical Pearl / High-Yield Fact**
It's essential to note that spermicides are most effective when used in combination with other forms of contraception, such as condoms or hormonal methods. Additionally, spermicides should be used in accordance with the manufacturer's instructions to ensure optimal effectiveness.

**Correct Answer:** D. Nonoxynol-9 disrupts sperm cell membrane by denaturing surface proteins, leading to immobilization and death.