Chapter 2 of 427% of exam

Diagnose: Differential Diagnosis and the Primary Diagnosis

Diagnose asks you to synthesize the history, examination, and test results into a prioritized differential and a primary diagnosis. Questions usually give a classic cluster of findings and ask for the most likely diagnosis, or a set of values and ask how they are classified. The skill tested is recognizing which finding separates one diagnosis from its closest look-alike.

What the domain covers

NPCB lists two tasks for Diagnose: synthesizing subjective and objective data into differential diagnoses, and prioritizing that differential to establish the primary diagnosis. The applied knowledge areas behind it are anatomy, physiology, pathophysiology, comorbidities, and diagnostic reasoning. Classification questions, such as blood pressure or glucose categories, sit here because they turn numbers into a diagnosis.

Two Diagnose tasks

Develop a differential from subjective and objective data, then prioritize it and establish the primary diagnosis.

NPCB FNP exam page, Domains and Tasks

Blood pressure categories

Normal is below 120/80; elevated is 120-129 systolic with diastolic below 80; stage 1 is 130-139 or 80-89; stage 2 is 140 or higher or 90 or higher.

NHLBI High Blood Pressure; 2025 AHA/ACC High Blood Pressure Guideline

Glucose categories

Prediabetes is an A1C of 5.7-6.4% or fasting glucose of 100-125 mg/dL; diabetes is an A1C of 6.5% or more, fasting glucose of 126 or more, 2-hour OGTT of 200 or more, or random glucose of 200 or more with symptoms.

NIDDK Diabetes Tests & Diagnosis (ADA criteria)

Infants, children and adolescents

Pediatric diagnoses are often defined by age of onset plus one signature finding. Knowing the typical age window is frequently what separates two conditions that share a symptom such as vomiting, limp, or rash.

Pyloric stenosis vs intussusception

Pyloric stenosis causes forceful nonbilious vomiting starting around 3 weeks with an olive-shaped mass; intussusception causes colicky pain and currant-jelly stool, usually between 5 months and 3 years.

MedlinePlus (NLM) Medical Encyclopedia

Limp and knee pain

In an adolescent with obesity, knee pain with a limp may come from slipped capital femoral epiphysis; a tender tibial tubercle in an active adolescent suggests Osgood-Schlatter disease.

MedlinePlus (NLM) Medical Encyclopedia

Childhood rashes

Slapped cheeks with a lacy body rash suggest fifth disease; a sandpaper rash with strep pharyngitis is scarlet fever; honey-colored crusts are impetigo.

MedlinePlus (NLM) Medical Encyclopedia

Airway illnesses

Barking cough and inspiratory stridor point to croup; wheezing and crackles in an infant under 2 after a cold point to bronchiolitis, most often from RSV.

MedlinePlus (NLM) Medical Encyclopedia; CDC pediatric outpatient guidance

Reproductive and sexual health

Gynecologic and STI diagnoses depend on a few decisive findings, such as vaginal pH, microscopy, pregnancy status, and pelvic tenderness. A positive pregnancy test changes the whole differential for pelvic pain.

Bacterial vaginosis

Amsel criteria are thin homogeneous discharge, clue cells, vaginal pH above 4.5, and a positive whiff test; at least three are needed.

CDC STI Treatment Guidelines 2021

PID minimum criteria

In a sexually active young woman with pelvic pain and no other cause, cervical motion, uterine, or adnexal tenderness is enough to start empiric treatment.

CDC STI Treatment Guidelines 2021

Ectopic pregnancy

Pain in early pregnancy with shoulder pain, weakness, or signs of bleeding suggests rupture and is an emergency.

ACOG patient FAQ, Ectopic Pregnancy

Menopause, PCOS, endometriosis

Menopause is confirmed after 12 months without a period; PCOS combines infrequent periods with androgen excess; endometriosis causes worsening dysmenorrhea, deep dyspareunia, and infertility.

ACOG; Office on Women's Health (HHS)

Adult and older adult conditions

For adults, the classic confusions are between inflammatory and degenerative joint disease, between thyroid overactivity and underactivity, and between transient and fixed neurologic deficits. Older adults often present with common conditions in which one feature, such as rest tremor or an irregularly irregular pulse, decides the diagnosis.

RA vs OA vs gout

Rheumatoid arthritis is symmetric with morning stiffness over 30 minutes; osteoarthritis stiffness usually lasts under 30 minutes; gout flares start suddenly, often at night in the big toe.

NIAMS Rheumatoid Arthritis, Osteoarthritis, Gout

Thyroid

High TSH with low free T4 is primary hypothyroidism, and TPO antibodies point to Hashimoto disease; Graves disease is the most common cause of hyperthyroidism.

NIDDK Hashimoto's Disease; MedlinePlus Hyperthyroidism

TIA

Stroke-like symptoms that usually resolve within an hour (up to 24 hours) define a transient ischemic attack and need urgent evaluation.

NINDS Transient Ischemic Attack

Parkinson disease vs atrial fibrillation clues

A pill-rolling tremor most obvious at rest suggests Parkinson disease; an irregularly irregular pulse without P waves is atrial fibrillation.

NINDS Parkinson's Disease; NHLBI Atrial Fibrillation

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