## **Core Concept**
The question pertains to the process of **pellicle formation** on teeth after cleaning. The acquired pellicle is a thin, proteinaceous film that forms on tooth surfaces shortly after brushing. It plays a critical role in protecting teeth and facilitating remineralization.

## **Why the Correct Answer is Right**
The correct answer, **A. Acquired pellicle**, is the first layer to form on tooth surfaces after brushing. This layer is composed primarily of **salivary proteins** such as **glycoproteins** and **enzymes** that adsorb onto the tooth enamel. The formation of the acquired pellicle is rapid, occurring within minutes after cleaning, and it serves as a foundation for further plaque formation.

## **Why Each Wrong Option is Incorrect**
- **Option B: Dental plaque** - This is a biofilm that forms on teeth after the acquired pellicle has been established. Plaque accumulation occurs over hours to days and consists of bacteria, their products, and extracellular matrix.
- **Option C: Calculus (Tartar)** - This is a hardened deposit that forms on teeth when plaque is not removed and mineralizes over time. Calculus formation requires weeks to months.
- **Option D: Gingivitis** - This is an inflammatory condition of the gingiva (gums) caused by poor oral hygiene leading to plaque accumulation and bacterial invasion. Gingivitis is a clinical condition rather than a layer formed on teeth.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **acquired pellicle** can reform rapidly, within minutes, after tooth brushing. This has implications for the timing of fluoride application and other dental treatments aimed at preventing caries and strengthening tooth enamel.

## **Correct Answer:** . Acquired pellicle