**Question:** A 24-year-old woman is referred to the clinic for assessment of a low white-cell count. She has no past medical history and is not on any medications. Her only symptoms are of joint discomfort in her hands, and occasional sharp chest pains that change with breathing. On physical examination, there is inflammation of some MCP and DIP joints in both hands, and the rest of the examination is normal. Her WBC is 3500/mL and on the differential the lymphocytes are low (15%) and PMNs are normal. Which of the following is the most likely diagnosis?

A. Lymphoma
B. Leukopenia
C. Autoimmune disease
D. Infectious disease

**Correct Answer:** **C. Autoimmune disease**

**Core Concept:**

A low white blood cell (WBC) count is a feature observed in various medical conditions, including leukopenia, lymphoma, autoimmune disease, and infectious disease. A differential count indicates the percentage of different types of white blood cells in the blood sample. In this scenario, the patient presents with a low WBC count (3500/mL) and low lymphocytes (15%) with normal polymorphonuclear neutrophils (PMNs).

**Why the Correct Answer is Right:**

The patient's clinical presentation aligns more with an autoimmune disease than other options. The low lymphocytes on the differential count suggests a decrease in the production of lymphocytes in the bone marrow, which is a characteristic feature in autoimmune diseases. In an autoimmune disease, the immune system mistakenly attacks the body's own tissues, including those of the bone marrow, leading to decreased WBC production.

**Why Other Options are Incorrect:**

A. Lymphoma (Option A) is a type of cancer that primarily affects lymphocytes. A low lymphocyte count in a lymphoma patient would not be associated with low polymorphonuclear neutrophils (PMNs).

B. Leukopenia (Option B) is a specific condition characterized by a low WBC count. While the patient has leukopenia, the percentage of lymphocytes is normal, making this option less likely.

D. Infectious diseases (Option D) might cause a decreased WBC count, but they tend to elevate lymphocytes rather than decrease them due to the immune system's response to pathogens. In this case, the normal PMN count further narrows down the diagnosis to an autoimmune disorder.

**Clinical Pearl:**

A low lymphocyte count in the context of joint inflammation and chest pain might suggest an autoimmune disorder, particularly rheumatoid arthritis or systemic lupus erythematosus. Autoimmune diseases can affect various organs and systems. In the presented case, the low lymphocytes and normal polymorphonuclear neutrophils (PMNs) indicate an autoimmune process targeting lymphocytes in the bone marrow, which is a hallmark of autoimmune disorders.

**Explanation of the Wrong Answer Options:**

A. Lymphoma is a type of cancer affecting lymphocytes. In this case, the normal polymorphonuclear neutrophils (PMNs) make lymphoma less likely as the primary diagnosis.

B. Leukopenia is a condition characterized by