Identifying WBC by size and nucleus features

Steps:

• Recall white blood cell types and their microscopic traits: size, shape, and nucleus.
• Note the description: very large diameter indicates the biggest WBC type.
• Identify U-shaped (kidney-shaped) nucleus occupying half the cell volume as a key feature.
• Match to monocyte, excluding non-WBCs and mismatched types.

Why A is correct:

• Monocytes are the largest leukocytes (12-20 μm diameter) with a U-shaped or indented nucleus filling about half the cytoplasm, per standard hematology definitions.

Why the others are wrong:

• B: Neutrophils are smaller (10-12 μm) with multi-lobed nuclei, not U-shaped.
• C: Red blood cells are non-nucleated and smaller (7-8 μm biconcave discs).
• D: Lymphocytes are small (7-10 μm) with round, dense nuclei occupying most of the cell.

Final answer: A