A 4-year-old female is being evaluated for the sudden onset of multiple petechiae and bruises. She is found to have a peripheral leukocyte count of 55,000, 86% of which are small, homogeneous cells that have nuclei with immature chromatin. Indistinct nucleoli are also present. Initial tests on these immature cells are as follows: TdT, positive; PAS, positive; acid phosphatase, positive; and myeloperoxidase, negative. Based on these findings, the immature cells most likely originated from

**Core Concept**
The question requires identification of a specific type of immature cell based on its morphological and biochemical characteristics. The key features include a high peripheral leukocyte count, predominantly small cells with immature chromatin, and specific enzyme staining patterns.

**Why the Correct Answer is Right**
The presence of TdT (terminal deoxynucleotidyl transferase) positivity, PAS (periodic acid-Schiff) positivity, and acid phosphatase positivity, along with myeloperoxidase negativity, is characteristic of lymphoblasts. TdT is an enzyme involved in DNA replication and is commonly found in lymphoid cells. PAS positivity suggests the presence of glycogen or other carbohydrates within the cell. Acid phosphatase positivity is also typical of lymphoid cells. Myeloperoxidase negativity rules out a myeloid origin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that TdT positivity is a key marker for lymphoblastic leukemia, especially in pediatric patients. This enzyme is often used in combination with other markers to diagnose and differentiate lymphoid malignancies.

**Correct Answer: D. Lymphoblasts.**

✓ Correct Answer: A. Lymphoblasts