**Question:** Three months ago, a 53 year old woman had surgery and chemotherapy for breast cancer. Four weeks ago, she developed a cough occasionally productive of purulent sputum. About 2 weeks ago, she noted a slight but progressive weakness of her left arm and leg. On chest examination, rales were heard over the left upper back when the patient breathed deeply. Neurologic examination confirmed weakness of the left arm and leg. Chest radiography showed a left upper lobe infiltrate. Contrast enhanced computed tomography showed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram positive rods that were partially acid fast. Which of the following organisms is the cause of this patient's current illness?

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Mycobacterium tuberculosis
D. Pseudomonas aeruginosa

**Correct Answer:** C. Mycobacterium tuberculosis

**Core Concept:**

The correct answer, Mycobacterium tuberculosis, is a bacterium responsible for causing tuberculosis (TB), primarily affecting the lungs but can also involve other organs. Tuberculosis is a common infectious disease caused by the bacillus, Mycobacterium tuberculosis. It primarily affects the lungs but can also spread to other parts of the body via the bloodstream.

**Why the Correct Answer is Right:**

In this case, the patient presents with symptoms such as cough, productive sputum, and neurological symptoms (weakness of the left arm and leg). The gram stain shows branching Gram-positive rods that are partially acid-fast, a characteristic feature of Mycobacterium tuberculosis.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus aureus (Staph): This is a common Gram-positive bacterium responsible for various infections, but typically does not exhibit partial acid-fastness.

B. Streptococcus pneumoniae (Pneumococcus): This bacterium causes pneumonia and is also typically not acid-fast.

D. Pseudomonas aeruginosa (Pseudomonas): This is a Gram-negative bacterium primarily associated with nosocomial infections and is not the cause of tuberculosis.

**Clinical Pearls:**

Tuberculosis remains a significant health concern worldwide, particularly in developing countries. It is crucial to consider this diagnosis in patients with respiratory symptoms, productive cough, and neurological symptoms, especially if the sputum Gram stain shows branching Gram-positive rods that are partially acid-fast. Early diagnosis and treatment of tuberculosis are essential for effective management and prevention of complications.