Source / episode info
- **Episode:**560
- **Title:**DIP Ep 560: 2024 USMLE Step 3 Free 137 Discussion Part 7 (Q61-70, super helpful for Step 2!)
- **Published:**2025-01-14
- Source:Episode page
One-liner
This episode integrates neuroanatomy (Horner syndrome/Pancoast tumor), infectious disease (Cryptosporidium/Immunodeficiency workup), biostatistics (ROC curve interpretation), and neuromuscular pathology (Muscular Dystrophy).
High-yield summary
- Horner Syndrome: The triad of ptosis, miosis (constricted pupil), and anhidrosis (sweating loss) suggests sympathetic chain damage. A common cause in smokers is a superior sulcus tumor (Pancoast tumor).
- Cryptosporidium: This protozoan causes watery diarrhea, and its oocysts are characteristically acid-fast positive. It is the most likely pathogen causing chronic watery diarrhea in an immunocompromised host (e.g., HIV).
- Immunodeficiency Workup: Recurrent infections (especially GI) suggest a primary immunodeficiency. The initial workup should focus on quantifying immunoglobulin levels (IgG, IgA, IgM) to rule out Common Variable Immunodeficiency (CVID) or specific deficiencies like IgA deficiency.
- ROC Curves: On an ROC curve, the Y-axis is Sensitivity and the X-axis is 1 - Specificity. A test that is more left-shifted has higher specificity; a test that is vertically elevated has higher sensitivity.
- Muscular Dystrophy: Proximal muscle weakness combined with calf pseudohypertrophy and elevated CK strongly suggests a muscular dystrophy, such as Duchenne Muscular Dystrophy (DMD).
Learning objectives
- Identify the clinical signs and underlying pathology of Horner syndrome, particularly in the context of lung masses.
- Differentiate between common causes of watery diarrhea using stool microscopy and staining techniques.
- Interpret ROC curves to determine which diagnostic test offers superior sensitivity or specificity based on clinical need.
- Recognize the classic physical exam findings (e.g., pseudohypertrophy) associated with muscular dystrophies.
- Formulate a systematic approach to diagnosing primary immunodeficiency following recurrent infections.
Board exam buzzwords