Master the board step 3

Infectious Diseases
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Adverse effects of amphotericin—renal toxicity (increased creatinine);
hypokalemia; metabolic acidosis; fever, shakes, chills
Basic Science Correlate
Mechanism of Renal Toxicity of Amphotericin
Amphotericin is directly toxic to the tubules. Distal tubule toxicity results in
renal tubular acidosis. Distal RTA gives excess potassium and magnesium loss
and hydrogen ion retention. When renal toxicity is described, the answer is
“Switch to liposomal amphotericin.”
Osteomyelitis
Is the infection in the soft tissue (skin) only? Or has it spread into the bone?
Osteomyelitis in adults almost always presents in a patient with diabetes,
peripheral arterial disease, or both with an ulcer or soft tissue infection. You
can also think about osteomyelitis in patients with direct trauma and a history
of orthopedic surgery, but the case with diabetes and peripheral vascular
disease is more likely to show up on the exam. The “Next best step?” question
fundamentally asks if you know what to do to distinguish the difference
between a soft tissue infection and a contiguous spread into the bone.
Diagnostic Testing
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Best initial test: Plain x-ray
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Best second test (if there is clinical suspicion and x-ray is negative): MRI
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Most accurate test: bone biopsy and culture
You must lose more than 50 percent of the calcium content of the bone before
the x-ray becomes abnormal. Although it may take up to two weeks for osteomyelitis
to become severe enough to make the x-ray abnormal, the x-ray is still
the first test. (You would not skip an EKG and go straight to a stress test; you
would not skip a chest x-ray and go straight to a chest CT scan.)
Which of the following is the earliest finding of osteomyelitis on an x-ray?
a. Periosteal elevation
b. Involucrum
c. Sequestrum
d. Punched-out lesions
e. Fracture
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