DIP Episode 597 - 2025 USMLE Step 1 Free 120 Discussion Part 12 (Q111-119, final part, super helpful for Step 2 and 3!)

Source / episode info

**Episode:**597
**Title:**DIP Ep 597: 2025 USMLE Step 1 Free 120 Discussion Part 12 (Q111-119, final part, super helpful for Step 2 and 3!)
**Published:**2025-04-29
Source:Episode page

One-liner

This episode integrates concepts across multiple systems, covering the diagnosis of cholinergic toxidrome secondary to AChE inhibitor overdose; genetic risks associated with chromosomal inversions; acute sympathomimetic toxicity from amphetamines; mitochondrial disorders presenting as myopathy; autoimmune gastritis leading to B12 deficiency; antiarrhythmic drug mechanisms (K+ blockade); and biostatistical calculations for screening test accuracy.

High-yield summary

Cholinergic Toxidrome: Overdose of acetylcholinesterase inhibitors (Pyridostigmine) leads to excessive acetylcholine, causing SLUDGE-like symptoms (salivation, lacrimation, urination, diarrhea, GI upset) and muscle fasciculations/weakness.
Chromosomal Inversions: A paracentric inversion increases the risk of early spontaneous abortions because crossing over within the inverted segment can lead to unbalanced gametes, but it does not increase the risk of aneuploidy (e.g., Trisomy 21).
Methotrexate Rescue: High-dose Methotrexate inhibits dihydrofolyl reductase (DHFR); rescue therapy requires a folic acid analog (like Leucovorin/Folinic Acid) to bypass this inhibited step and restore DNA synthesis.
Pernicious Anemia: Autoantibodies target the parietal cells of the stomach, leading to loss of intrinsic factor production and subsequent Vitamin B12 deficiency.
Neutropenic Fever: Any fever in a patient undergoing chemotherapy (especially for ALL) must be presumed due to neutropenia and requires immediate empirical broad-spectrum antibiotics (e.g., anti-pseudomonal coverage).

Learning objectives

Differentiate the clinical signs and pathophysiology of cholinergic toxidrome from other causes of muscle weakness (e.g., MG exacerbation).
Interpret chromosomal abnormalities, specifically distinguishing between inversions/translocations and aneuploidy risks.
Identify the mechanism of action for sympathomimetic drugs like amphetamines and their resulting toxicities.
Recognize the classic findings (RRFs) and underlying pathophysiology of mitochondrial myopathies.
Correlate autoimmune gastritis with B12 deficiency, identifying the specific target cell (parietal cells).
Understand the critical management principles for neutropenic fever in oncology patients.