The PHARYNGEAL & AORTIC
Derivatives
(Arches,Pouches,Clefts)
nearly broke me during Step 1 prep❌

Then I saw the cranial nerve pattern (5→ 7→ 9→10) & it all just made sense✅

Here’s a complete breakdown🧵🧵

(You won’t need to read anything else for this topic - Save this) CLEFT ->Ectoderm
ARCH ->Mesoderm
POUCH->Endoderm

1️⃣ CN V (Trigeminal) – “Chew”

All derivatives are related to the Jaw.

✅→ 1st Pharyngeal Arch (Mesoderm)= Muscles, Bones, Arteries

🔹 Muscles: Muscles of mastication (Masseter, Temporalis, Medial/Lateral Pterygoid), Mylohyoid, Anterior Digastric, Tensor Tympani, Tensor Veli Palatini.
🔹 Bones: Maxilla, Mandible, Zygomatic, Incus & Malleus.

✅—> 1st Pharyngeal Pouch (Endoderm)

🔹Middle ear cavity, Eustachian tube.

✅—> 1st Pharyngeal Cleft
(Ectoderm)
🔹 External Auditory Meatus (EAM).

🔴 1st Aortic Arch Artery:
Maxillary artery

🔥 Clinical Correlation:
CN V3 damage →Jaw deviation TOWARD lesion (unopposed pterygoid muscle).

•Internal vs. External Validity ✅

•Lead vs Length Time Bias ⏳

• Precision vs. Accuracy🎯

Some of the most HY yet painfully tricky Biostats concepts on the USMLE.

Here is the clearest breakdown with analogies you’ll never forget.
🧵👇🏻 Internal vs. External Validity

The simplest breakdown of this concept!

Late addition to this thread
(Just remembered how much of a headache these 2 were and were even asked in my real Step Exam)

✅ Internal Validity =
“Did we do the study right?”

High internal validity = Well-designed, controlled, no bias.

Low internal validity = Confounding, bias, poor controls → Unreliable results.

📌 Example:

A study on a new drug has randomized groups, proper blinding, and controlled variables → High internal validity (results are trustworthy).

A study with selection bias, confounders, or poor blinding → Low internal validity (we don’t know if the drug actually works).

🚨 Contrast with external validity:

If this study was done only in healthy young males, it has high internal validity (well-controlled), but low external validity (can’t apply to older adults, women, diabetics, etc.).

If the study includes a diverse population, it has higher external validity (more generalizable).

📌 How to improve internal validity?
✔ Randomization (removes selection bias)
✔ Blinding (removes experimenter bias)
✔ Controlling confounders (ensures the effect is due to the treatment)

🌍 External Validity =
“Can we apply this to real life?”

High external validity = The study applies to a wide range of patients.

Low external validity = The study only works for a specific group.

📌 Example:

A new hypertension drug is tested only in young, healthy males → Low external validity (can’t generalize to all patients).

A study includes a diverse population (age, gender, comorbidities) → High external validity (results apply to real-world patients).

🚨 Contrast with internal validity:

A diverse study may have higher external validity but slightly lower internal validity due to more confounders.

A tightly controlled study (healthy males only) has high internal validity but low external validity.

📌 Other Key Biostats Terms:

🔹 Statistical Significance (p < 0.05) → Affects internal validity (ensures results aren’t by chance).

🔹 Clinical Significance → Affects external validity (ensures results actually matter in real life).

🔹 Confounding Variables & Bias → Lower internal validity
(mess up study accuracy).

🔑 Takeaway: A study with high internal validity but low external validity is well-done but useless for real patients.

HY USMLE Step 1 CVS.

cAMP vs. cGMP – Heart & Vessels Made Simple 🫀

During my Step1 preparation 📝

Understanding where
cAMP causes Contraction vs Relaxation,

Function of cGMP,etc
all of this was
Super IMPORTANT
Yet very confusing🤕

So Let’s break it down🧵👇🏻 cAMP

🫀 Heart:

• β₁ → Gs → ↑ cAMP → PKA → ↑ Ca²⁺ → ↑ HR & Contractility

• ↑ cAMP = Stronger & Faster Heartbeat

🩸 Vessels:

• β₂ → Gs → ↑ cAMP → PKA → ↓ MLCK → Vasodilation

• cAMP → Smooth muscle relaxation → ↓ TPR

🍽️ GI:

• Gs → cAMP → PKA → ↓ MLCK → Smooth muscle relaxation

• VIP → cAMP → ↑ Secretions & Relaxation → Helps digestion

📌 Clinical Tip:

Cholera & ETEC toxins: Overactivate Gs → ↑ cAMP → ↑ Cl⁻ & H₂O loss → Secretory diarrhea

USMLE Biostats was a headache for me.

Especially topics like

• Likelihood Ratios
• Predictive Values
• Sensitivity/Specificity.
• NNT, ARR, RR, OR , etc

Below lie the best explanations of these topics.

(This is NOT an aggregation of Formulae)

🧵🧵🧵🧵 Sensitivity vs. Specificity

Sensitivity and Specificity measure the TEST itself,
NOT the population.

•Sensitivity: The probability that the TEST will be positive,
when the patient has the disease.

•Example: 90% sensitivity = 90% of people +ve for the disease,
will TEST positive.

•Specificity: The probability that the TEST will be negative when the patient does not have the disease.

•Example: 90% specificity = 90% of people without the disease will TEST negative.

Unaffected by prevalence.

Now that the basics are clear,

Here’s a Neil Randy Video
To execute the Formula for MCQs

Sensitivity- youtube.com/shorts/9iQKw7Z…

Specificity- youtu.be/bv1MIMWecC8?si…

I studied 5.5 months for my Step2CK 📚

Scored 254 📈

Here’s a list of all the Super High Yield Stuff for
GIT🍕🍟🍔

That you should know before sitting for your exam!!

(⭐️->That topic was asked on my
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•Femoral Hernia always Surgery!(HY)

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I studied 4 months for
USMLE STEP 1 📚
Passed in November 2023.✅

Here’s a list of all the resources, that I used to SPEED UP my
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I studied 5 months for
My USMLE Step2 CK
Score: 254.

Here’s a detailed list of all the Super High Yield
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#Step2CK 1- For Public Health Sciences( Ethics and Biostats )
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Cognitive Errors (530)
Quality Improvement ( 230)
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Biases (275,458)
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Drug Ads (337)

All are to the Point and Super Interesting !