A 13-year-old girl is referred to an oral surgeon after complaining of tooth pain, especially in the upper jaw. A review of her medical history reveals status post-surgical repair of a patent ductus arteriosus when she was 6 years old. At the clinic, her temperature is 37.0ºC (98.6°F), pulse is 90/min, respirations are 18/min, and blood pressure is 110/78 mm Hg. On physical examination, her height is 157.5 cm (5 ft 2 in), her weight is 50 kg (110 lb) and her arms seem disproportionately long for her trunk. She also has arachnodactyly and moderate joint laxity. Intraoral examination reveals crowded teeth and a high arched palate. Which of the following protein is most likely defective in this condition?

Caveolin and the sarcoglycan proteins

Cardiac muscle serves many necessary functions, leading to a specific structure that serves these functions. The structure highlighted is an important histology component of cardiac muscle. What would be the outcome if this structure diffusely failed to function?

Failure of propagation of the action potential from the conduction system

Failure of potassium channels to appropriately open to repolarize the cell

Ineffective excitation-contraction coupling due to insufficient calcium ions

Inappropriate formation of cardiac valve leaflets

A healthy 22-year-old male participates in a research study you are leading to compare the properties of skeletal and cardiac muscle. You conduct a 3-phased experiment with the participant. In the first phase, you get him to lift up a 2.3 kg (5 lb) weight off a table with his left hand. In the second phase, you get him to do 20 burpees, taking his heart rate to 150/min. In the third phase, you electrically stimulate his gastrocnemius with a frequency of 50 Hz. You are interested in the tension and electrical activity of specific muscles as follows: Biceps in phase 1, cardiac muscle in phase 2, and gastrocnemius in phase 3. What would you expect to be happening in the phases and the respective muscles of interest?

Increase of tension in all phases

Increase of tension in experiments 2 and 3, with the same underlying mechanism

Recruitment of large motor units followed by small motor units in experiment 1

Fused tetanic contraction at the end of all three experiments

Recruitment of small motor units at the start of experiments 1 and 2

A 57-year-old man comes to the physician for a follow-up evaluation of chronic, retrosternal chest pain. The pain is worse at night and after heavy meals. He has taken oral pantoprazole for several months without any relief of his symptoms. Esophagogastroduodenoscopy shows ulcerations in the distal esophagus and a proximally dislocated Z-line. A biopsy of the distal esophagus shows columnar epithelium with goblet cells. Which of the following microscopic findings underlie the same pathomechanism as the cellular changes seen in this patient?

Squamous epithelium in the bladder

Branching muscularis mucosa in the jejunum

Pseudostratified columnar epithelium in the bronchi

Disorganized squamous epithelium in the endocervix

A 65-year-old man with a 40-pack-year smoking history presents with hemoptysis and a persistent cough. Chest CT shows a 3.5 cm centrally located mass in the right main bronchus. Positron emission tomography confirms a malignant nodule. Bronchoscopy with transbronchial biopsy is performed and a specimen sample of the nodule is sent for frozen section analysis. The tissue sample is most likely to show which of the following tumor types?

Metastasis of colorectal cancer

Cell and tissue identification keys for USMLE Step 3: High-Yield Anatomy Lessons

Epithelial Tissues - Lining Up Cells

Classification: Based on number of layers (simple, stratified) and cell shape (squamous, cuboidal, columnar).

Key Types & Locations:
- Simple Squamous: Endothelium, alveoli (gas exchange).
- Simple Columnar: GI tract (absorption).
- Stratified Squamous: Skin, esophagus (protection).
- Pseudostratified Ciliated Columnar: Trachea, bronchi (mucociliary clearance).
- Transitional: Bladder, ureters (distensibility).

Epithelial Tissue Types

⭐ Barrett's Esophagus: Metaplasia of the lower esophagus where stratified squamous epithelium is replaced by mucus-secreting simple columnar epithelium due to chronic GERD.

Connective Tissues - The Body's Glue

Components: Cells + Extracellular Matrix (ECM).
- ECM = Ground Substance (GAGs, proteoglycans) + Fibers.
Key Cells: Fibroblasts (synthesize ECM), adipocytes (storage), mast cells (histamine), macrophages (phagocytosis).
Fibers:
- Collagen: Main structural protein. 📌 Strong, Cartilage, Bloody, Basement Membrane (Types I, II, III, IV).
- Elastin: Provides stretch and recoil (e.g., aorta, skin).
- Reticulin (Type III Collagen): Forms delicate meshwork (e.g., liver, bone marrow).

Areolar Connective Tissue: Diagram and Histology

⭐ Marfan Syndrome: A defect in Fibrillin-1 (FBN1 gene) impairs the structural integrity of elastin fibers, leading to systemic issues in the skeleton, eyes, and cardiovascular system.

Muscle Tissues - Movers and Shakers

Feature	Skeletal	Cardiac	Smooth
Striations	Yes	Yes	No
Nuclei	Multi, peripheral	1-2, central	Single, central
Control	Voluntary	Involuntary	Involuntary
Cell Shape	Cylindrical	Branched	Fusiform
Key ID	Peripheral nuclei	Intercalated discs	No striations

⭐ Intercalated discs are unique to cardiac muscle, containing desmosomes (anchoring) and gap junctions (ionic coupling), allowing the myocardium to contract as a syncytium.

Nervous Tissues - The Control Freaks

Neurons: Excitable cells for signal transmission.
- Soma (Cell Body): Large nucleus, prominent nucleolus.
- Nissl Bodies: Basophilic granular clusters of RER/ribosomes in soma & dendrites; absent in axon hillock.
- Axon: One per neuron, transmits action potentials.
- Dendrites: Multiple, receive signals.
Glial Cells: Non-neuronal support cells.
- CNS: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells.
- PNS: Schwann cells, Satellite cells.

⭐ In the CNS, one Oligodendrocyte myelinates multiple axons. In the PNS, one Schwann Cell myelinates a single segment of one axon.

Types of Neuroglia in CNS and PNS

High‑Yield Points - ⚡ Biggest Takeaways

Epithelium: Look for the basement membrane and intercellular junctions. Classification is based on layers and cell shape.

Connective Tissue: Characterized by abundant extracellular matrix (ECM) with fewer cells.

Muscle: Eosinophilic cytoplasm. Striations identify skeletal/cardiac muscle; intercalated discs are unique to cardiac.

Nervous Tissue: Find large neurons with Nissl substance and surrounding smaller glial cells.

Key Identifiers: Goblet cells (mucus), cilia (motility), and nucleus-to-cytoplasm ratio (high in cancer/lymphocytes).

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