Regarding the pathophysiology and clinical features of acute osteomyelitis, all of the following are true EXCEPT:

Explanation

## Pathophysiology of Acute Osteomyelitis ### Causative Organisms **Key Point:** Staphylococcus aureus accounts for 80–90% of cases in both children and adults, followed by Streptococcus pyogenes and Gram-negative organisms (especially in neonates and immunocompromised hosts). [cite:Robbins 10e Ch 28] ### Site Predilection in Children **High-Yield:** The metaphyseal region is most commonly affected because: - Slow blood flow in sinusoidal capillaries at the metaphysis - Capillaries make sharp turns, promoting bacterial seeding - Absence of phagocytic lining in these vessels - This explains why metaphyseal osteomyelitis is the hallmark in skeletally immature patients ### Pathological Progression **Clinical Pearl:** Subperiosteal abscess formation is a characteristic early finding: 1. Bacterial infection triggers acute inflammation 2. Increased intramedullary pressure 3. Pus tracks along Haversian canals to the periosteum 4. Periosteum lifts, creating a subperiosteal abscess 5. This occurs BEFORE sequestrum (dead bone) formation ### Laboratory Findings — The Trap **Warning:** Elevated alkaline phosphatase and hypocalcemia are NOT early or specific findings in acute osteomyelitis. The earliest laboratory changes are: - **Elevated ESR and CRP** (non-specific but sensitive; rise within hours) - **Elevated WBC count** (left shift) - **Blood cultures** (positive in ~50% of cases) - **Elevated alkaline phosphatase** is a LATE finding (weeks), reflecting osteoblastic repair and new bone formation, not acute infection - **Hypocalcemia** is rare and not a feature of acute osteomyelitis **Key Point:** ESR and CRP are the most useful early laboratory markers, not alkaline phosphatase or calcium levels. ## Summary Table | Feature | True/False | Notes | | --- | --- | --- | | S. aureus is most common | **TRUE** | 80–90% of cases | | Metaphyseal predilection in children | **TRUE** | Due to vascular anatomy | | Subperiosteal abscess is early sign | **TRUE** | Occurs before sequestrum | | ↑ Alk Phos & ↓ Ca are earliest findings | **FALSE** | Alk Phos is late; hypocalcemia is rare |