Nail-Patella Syndrome

Nail-patella syndrome is a genetic disease of the connective tissue that produces defects in the fingernails, bone joints, and kidneys.


Narcolepsy is a disorder of the nervous system marked by excessive daytime sleepiness, uncontrollable sleep attacks, and cataplexy (a sudden loss of muscle tone, usually lasting up to half an hour). The American Psychiatric Association (APA) classifies narcolepsy as a sleep disorder in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, or DSM-IV.

Narcotic Drugs

Narcotics are addictive drugs that reduce the user's perception of pain and induce euphoria (a feeling of exaggerated and unrealistic well-being). The English word narcotic is derived from the Greek narkotikos, which means "numbing" or "deadening." Although the term can refer to any drug that deadens sensation or produces stupor, it is commonly applied to the opioids—that is, to all natural or synthetic drugs that act like morphine.

Nasal Trauma

Nasal trauma is defined as any injury to the nose or related structure that may result in bleeding, a physical deformity, a decreased ability to breathe normally because of obstruction, or an impaired sense of smell. The injury may be either internal or external.

Nausea and Vomiting

Nausea is the sensation of being about to vomit. Vomiting, or emesis, is the expelling from the stomach of undigested food through the mouth.


Near-drowning is the term for survival after suffocation caused by submersion in water or other fluid. Some experts exclude from this definition cases of temporary survival that end in death within 24 hours, which they prefer to classify as drownings.

Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is a serious bacterial infection in the intestine, primarily affecting sick or premature newborn infants. It can cause the death (necrosis) of intestinal tissue and progress to blood poisoning (septicemia).

Neonatal Jaundice

Neonatal jaundice is the term used when a newborn has an excessive amount of bilirubin in the blood. Bilirubin is a yellowish-red pigment that is formed and released into the bloodstream when red blood cells are broken down.

Neonatal Reflexes

Neonatal reflexes or primitive reflexes are the inborn behavioral patterns that develop during uterine life. They should be fully present at birth and are gradually inhibited by higher centers in the brain during the first three to 12 months of postnatal life.


Neurofibromatosis (NF) is a genetic disease in which multiple soft tumors (neurofibromas) develop under the skin and throughout the nervous system. Various sized tumors may grow on the nerves in or leading away from the brain and spinal cord (peripheral nerves) and in the vascular system (veins and arteries) and other organ systems.

Neurologic Exam

A neurological examination is an essential component of a comprehensive physical examination. It is a systematic examination that surveys the functioning of nerves delivering sensory information to the brain and carrying motor commands (peripheral nervous system) and impulses back to the brain for processing and coordinating (central nervous system).

Night Terrors

Childhood night terrors are a parasomnia, or partial-sleep disorder, common in young children. They occur in the deepest stage of sleep and are characterized by an abrupt arousal, usually within the first hour of sleep.


Nightmares are a type of sleep disruption, or parasomnia, characterized by frightening psychological content. Nightmares provoke a feeling of imminent physical danger with a sensation of being trapped or suffocated.

Nonsteroidal Anti-Inflammatory Drugs

Nonsteroidal anti-inflammatory drugs are medicines that relieve pain, swelling, stiffness, and inflammation.


A nosebleed, also called epistaxis, is a loss of blood from any blood vessel in the nose. It usually appears in only one nostril.

Numbness and Tingling

Numbness and tingling are decreased or abnormal sensations caused by altered sensory nerve function.

Nursemaid's Elbow

Nursemaid's elbow is an injury to the ligament (strong band of tissue) that keeps the two bones of the forearm in the correct place.


The process by which humans take in and use food in their bodies; also the study of diet as it relates to health.


Nystagmus is a condition in which there is involuntary and rhythmic movement or oscillation of the eye. It is often caused by an underlying ocular or neurological disorder.


Obesity is an abnormal accumulation of body fat, usually 20 percent or more over an individual's ideal body weight. Obesity is associated with increased risk of illness, disability, and death.

Obsessive-Compulsive Disorder

Obsessive-compulsive disorder (OCD) is a type of anxiety disorder. Anxiety disorder is the experience of prolonged, excessive worry about circumstances in one's life.


Medical dictionaries define oligomenorrhea as infrequent or very light menstruation. But physicians typically apply a narrower definition, restricting the diagnosis of oligomenorrhea to women whose periods were regularly established before they developed problems with infrequent flow.

Oppositional Defiant Disorder

Oppositional defiant disorder (ODD) is a childhood mental disorder characterized by a pattern of angry, antagonistic, hostile, negative, irritable, and/or vindictive behavior lasting at least six months and occurring more frequently than is typically observed for the child's age and developmental stage. Children diagnosed with ODD do not meet the clinical diagnostic criteria for conduct disorder.

Oral Contraceptives

Oral contraceptives are medicines taken by mouth to help prevent pregnancy. They are also known as the pill, OCs, or birth control pills.

Oral Hygiene

Oral hygiene is the practice of keeping the mouth clean and healthy by brushing and flossing to prevent tooth decay and gum disease.


Orthodontics is a specialized branch of dentistry that diagnoses, prevents, and treats dental and facial irregularities called malocclusions. Orthodontics includes dentofacial orthopedics, which is used to correct problems involving the growth of the jaw.


The term osteochondroses refers to a group of diseases of children and adolescents in which localized tissue death (necrosis) occurs, usually followed by full regeneration of healthy bone tissue. The singular term is osteochondrosis.

Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a group of genetic diseases in which the bones are formed improperly, making them fragile and prone to breaking.


Osteopetrosis (plural osteopetroses) is a rare inherited disorder that makes bones increase in both size (mass) and fragility. It is a potentially fatal condition that can deform bone structure and distort the appearance.

Otitis Externa

Otitis externa refers to an infection of the ear canal (outer ear), the tube leading from the outside opening of the ear in towards the ear drum. The infection usually develops in children and adolescents whose ears are exposed to persistent, excessive moisture.

Otitis Media

Otitis media is an infection of the middle ear, which is located behind the eardrum. There are two main types of otitis media.


Overhydration, also called water excess or water intoxication, is a condition in which the body contains too much water.