**Core Concept**
The protein-lipid ratio in cell membranes is a critical determinant of membrane fluidity and function. This ratio is influenced by the fatty acid composition and the presence of cholesterol, which can modulate membrane fluidity.

**Why the Correct Answer is Right**
A decrease in the protein-lipid ratio is indicative of an increase in membrane fluidity. This occurs when there is an increase in the proportion of unsaturated fatty acids, which have a kinked structure that disrupts the fluidity of the membrane. Additionally, an increase in cholesterol can also lead to a decrease in the protein-lipid ratio by inserting into the membrane and disrupting the fluidity of the phospholipid bilayer. This is particularly relevant in the context of certain diseases, such as Alzheimer's disease, where an increase in membrane fluidity is thought to contribute to the pathogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a clear understanding of why the protein-lipid ratio would decrease.
**Option B:** This option is incorrect because it suggests a decrease in membrane fluidity, which is the opposite of what occurs with a decrease in the protein-lipid ratio.
**Option C:** This option is incorrect because it does not provide a clear explanation of how the protein-lipid ratio would decrease.

**Clinical Pearl / High-Yield Fact**
A decrease in the protein-lipid ratio can have significant implications for cellular function, particularly in the context of membrane-bound enzymes and receptors. For example, an increase in membrane fluidity can lead to a decrease in the activity of certain enzymes, which can have significant effects on cellular metabolism.

**Correct Answer:** A. Lecithin-sphingomyelin ratio