**Core Concept**
The Periodic Acid-Schiff (PAS) stain is a histological technique used to detect carbohydrate macromolecules such as glycogen, glycoproteins, and glycolipids in tissue sections. Block positivity in the context of PAS staining is a characteristic feature of certain types of cells.

**Why the Correct Answer is Right**
Lymphoblasts, the immature precursors of lymphocytes, exhibit block positivity with the PAS stain due to the presence of abundant glycogen in their cytoplasm. The PAS reaction involves the oxidation of carbohydrate moieties, resulting in a darkly staining block-like pattern in lymphoblasts. This is a distinctive feature that helps in the identification of these cells in bone marrow and lymphoid tissues. The presence of glycogen in lymphoblasts is related to their high metabolic activity and rapid cell division.

**Why Each Wrong Option is Incorrect**
**Option A:** Myeloblasts are the immature precursors of neutrophils and do not typically exhibit block positivity with the PAS stain. While they may contain some glycogen, the PAS reaction is not as intense or characteristic as seen in lymphoblasts.
**Option C:** Monoblasts, the precursors of monocytes, may show some PAS positivity but it is not typically block-like. Monoblasts tend to have a more diffuse or granular staining pattern with the PAS stain.
**Option D:** Megakaryoblasts, the precursors of megakaryocytes, do not typically exhibit block positivity with the PAS stain. Their cytoplasm may contain some glycogen but it is not a characteristic feature of these cells.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that block positivity with the PAS stain is a characteristic feature of lymphoblasts, which can be helpful in diagnosing acute lymphoblastic leukemia (ALL). This knowledge can aid in the accurate identification of these cells in bone marrow and lymphoid tissues.

**✓ Correct Answer: B. Lymphoblasts**