**Question:** Most common cause of osteomyelitis is -

A. Trauma
B. Infection
C. Cancer
D. Immune deficiency

**Core Concept:**
Osteomyelitis is a bacterial infection affecting the bone, typically involving the deep layers of the bone cortex. The disease is categorized into three types: (1) acute, (2) chronic, and (3) suppurative osteomyelitis. The most common cause of osteomyelitis is due to bacterial invasion, which leads to inflammation and tissue damage, causing symptoms like pain, redness, swelling, and warmth in the affected area.

**Why the Correct Answer is Right:**
The correct answer, B. Infection, refers to the primary cause of osteomyelitis, which is bacterial invasion. Bacteria can enter the bone through direct inoculation (e.g., trauma, surgical procedures), hematogenous spread (e.g., from another infected site), or contiguous spread (e.g., from an adjacent soft tissue infection). Once inside the bone, bacteria can cause inflammation and tissue damage, leading to the characteristic symptoms of osteomyelitis.

**Why Each Wrong Option is Incorrect:**
A. Trauma (choice A) is a risk factor for osteomyelitis but not the primary cause. Trauma facilitates bacterial inoculation into the bone but does not directly cause the infection.

C. Cancer (choice C) can weaken the immune system, making an individual susceptible to infections, including osteomyelitis. However, cancer itself does not directly cause osteomyelitis.

D. Immune deficiency (choice D) can increase the risk of osteomyelitis, as an impaired immune system would struggle to fight off bacterial infections. Similar to option C, immune deficiency does not directly cause osteomyelitis but rather a predisposition to it.

**Clinical Pearl:**
Understanding the different routes of bacterial entry into the bone is crucial for proper diagnosis and treatment of osteomyelitis. While trauma, cancer, immune deficiency, and infection are all associated with increased risk, only infection (choice B) directly causes the disease due to bacterial invasion and tissue damage.