**Question:** Thick PBS is made for:

A. Preservation of cells
B. Fixation of cells
C. Staining of cells
D. Immunostaining of cells

**Core Concept:** Thick Pap Smear (Pap smear or Pap test) is a test used in cervical cancer screening that involves collecting cells from the cervix. Thick Pap smear is one of the two types of Pap smears (the other being thin Pap smear) and is prepared by collecting cells onto a single slide rather than multiple slides.

**Why the Correct Answer is Right:** Thick Pap smear is made for the purpose of **fixation of cells**. Fixation is an essential step in preserving the cellular structure, morphology, and integrity of the cells collected for further processing and diagnostic evaluation. The fixative used in thick Pap smear is typically a combination of alcohols (e.g., 95% ethanol) with a fixative like Bouin's or Carnoy's solution. This ensures that the cells are well-preserved for subsequent staining and examination under the microscope.

**Why Each Wrong Option is Incorrect:**

A. **Preservation of cells:** While preserving cells is important, thick Pap smear is specifically designed for fixation, not preservation.

B. **Fixation of cells:** As mentioned earlier, thick Pap smear is primarily designed for fixation, not fixation of cells.

C. **Staining of cells:** Thick Pap smear is prepared for fixation, and staining is a subsequent step in the diagnostic process.

D. **Immunostaining of cells:** Immunostaining is a specific technique used for identifying cells expressing specific antigens, and thick Pap smear is not designed for this purpose.

**Core Concept:** Thick Pap smear is a valuable diagnostic tool for cervical cancer screening, and the correct answer, fixation, is essential for maintaining cell morphology and allowing for subsequent staining and diagnostic evaluation. The other options, including preservation, staining, and immunostaining, are not the primary purpose of a thick Pap smear.