**Question:** A patient presents with fever for 3 weeks. On examination, he is observed to have splenomegaly. Ultrasonography reveals a hypoechoic shadow in spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism

A. Escherichia coli
B. Klebsiella pneumoniae
C. Salmonella species
D. Pseudomonas aeruginosa

**Core Concept:**
Gram-negative bacilli are a group of bacteria that have a distinctive appearance under the microscope due to the presence of gram-negative cell walls. They are often associated with specific clinical presentations and disease states. In this case, the presence of fever, splenomegaly, and gram-negative bacilli on blood culture strongly suggests a bacterial infection.

**Why the Correct Answer is Right:**
The correct answer, D. Pseudomonas aeruginosa, is a gram-negative bacillus that is known to cause infections in immunocompromised patients and those with underlying conditions such as cystic fibrosis, diabetes, and chronic kidney disease. In this case, the patient's prolonged fever and splenomegaly are indicative of a severe infection, which is more commonly associated with Pseudomonas aeruginosa than the other options.

**Why Each Wrong Option is Incorrect:**
A. Escherichia coli (E. coli) is another gram-negative bacillus but is less likely to cause severe infections in immunocompromised patients, making it a less probable choice.
B. Klebsiella pneumoniae is another gram-negative bacillus, but it is less common in causing severe infections in immunocompromised patients.
C. Salmonella species are typically associated with food poisoning and gastrointestinal infections, rather than causing severe systemic infections in immunocompromised patients.

**Clinical Pearl:**
Infections caused by Pseudomonas aeruginosa often present with fever, splenomegaly, and other systemic manifestations due to their ability to invade host tissues and produce toxins. In immunocompromised patients, these bacteria can cause severe infections that may lead to sepsis and organ dysfunction.

**Core Concept:**
The presence of fever, splenomegaly, and gram-negative bacilli on blood culture strongly suggests a bacterial infection, particularly in immunocompromised patients. Gram-negative bacilli are known to cause severe infections due to their ability to invade host tissues and produce toxins.

**Why Each Wrong Option is Incorrect:**
A. E. coli and K. pneumoniae are common causes of bacterial infections, but they are less likely to cause severe infections in immunocompromised patients.
C. Salmonella species are typically associated with food poisoning and gastrointestinal infections, not causing severe systemic infections in immunocompromised patients.

**Clinical Pearl:**
In immunocompromised patients, infections caused by Pseudomonas aeruginosa can lead to sepsis and organ dysfunction due to their ability to invade host tissues and produce toxins.