A fifteen-month-old boy is rushed to the emergency department with sudden bluish lips and fingertips—but his lungs are clear, chest X-ray is normal, and he isn’t in respiratory distress. Oxygen saturation is low and barely improves with high-flow oxygen. A bedside blood draw reveals a strange chocolate-brown color.

What’s going on?

In this Master Series episode, we walk you through a classic exam-style vignette that challenges your ability to link history, physical exam, and next-step management. You’ll practice critical Step 2 CK reasoning, including:

– Interpreting low oxygen saturation in the context of normal lung findings
– Identifying key red flags that hint at a hidden diagnosis
– Understanding when oxygen won’t fix the problem—and what will
– Avoiding tempting but incorrect answer choices on the exam
– Using clinical judgment to make the next best move—even before labs come back

We won’t spoil the diagnosis up front—you’ll work through it like a real case. This is the kind of reasoning Step 2 CK rewards: quick thinking, pattern recognition, and decisive management.

Study smart. Stay sharp. And keep mastering your craft.