Pathology gives explanations of a disease by studying the following four aspects of the disease.

1. Etiology,

2. Pathogenesis,

3. Morphologic changes and

4. Functional derangements and clinical significance.

1. Etiology Etiology of a disease means the cause of the disease. If the cause of a disease is known it is called primary etiology. If the cause of the disease is unknown it is called idiopathic. Knowledge or discovery of the primary cause remains the backbone on which a diagnosis can be made, a disease understood, & a treatment developed. There are two major classes of etiologic factors: genetic and acquired (infectious, nutritional, chemical, physical, etc).

2. Pathogenesis Pathogenesis means the mechanism through which the cause operates to produce the pathological and clinical manifestations. The pathogenetic mechanisms could take place in the latent or incubation period. Pathogenesis leads to morphologic changes.

3. Morphologic changes The morphologic changes refer to the structural alterations in cells or tissues that occur following the pathogenetic mechanisms. The structural changes in the organ can be seen with the naked eye or they may only be seen under the microscope. Those changes that can be seen with the naked eye are called gross morphologic changes & those that are seen under the microscope are called microscopic changes. the morphologic changes will lead to functional alteration & to the clinical signs & symptoms of the disease.

4. Functional derangements and clinical significance The morphologic changes in the organ influence the normal function of the organ. By doing so, they determine the clinical features (symptoms and signs), course, and prognosis of the disease.

Seborrheic dermatitis is a scaly dermatitis on the scalp (dandruff) and face.
 - due to Pitysporium species
 - can be seen in AIDS as an opportunistic infection

SMALL INTESTINE 

Congenital anomalies 

1. Meckel's diverticulum (a true diverticulum) is due to persistence of the omphalomesenteric vitelline duct. 
2. Atresia is a congenital absence of a region of bowel, leaving a blind pouch or solid fibrous cord. 
3. Stenosis refers to a narrowing of any region of the gastrointestinal tract, which may cause obstruction. 
4. Duodenal diverticula are areas of congenital weakness permitting saccular enlargement. The duodenum is the most common region of the small bowel to contain diverticula. 
5. Diverticula of jejunum and ileum are herniations of mucosa and submucosa at points where the mesenteric vessels and nerves enter. 

Infections

1. Bacterial enterocolitis may be caused by the ingestion of preformed bacterial toxins, producing symptoms ranging from severe but transient nausea, vomiting, and diarrhea (Staphylococcus aureus toxin) to lethal paralysis (Clostridium botulinum toxin). Ingestion of toxigenic bacteria with colonization of the gut (e.g., Vibrio cholera, toxigenic E. coli, various species of Campylobacter jejuni, Shigella, salmonel
Yersinia, and many others) is another potential cause. 

2. Nonbacterial gastroenterocolitis
a. Viral 
(1) Rotavirus (children)
(2) Parvovirus (adults) 
b. Fungal-Candida 
c. Parasitic 
(1 ) Entamoeba histolytica 
(2) Giardia lamblia 

3. In HIV patients. Causes of infectious diarrhea in HIV patients include Cryptosporidium, Microsporidia, isospora belli, CMV, and M. avium-intracellulare. 

C. Malabsorption is defined as impaired intestinal absorption of dietary constituents. 
Clinical features include diarrhea,steatorrhea, weakness, lassitude, and weight loss. Steatorrhea results in deficiency of fat-soluble vitamins (A, D, E, K) and calcium. 

1. Celiac sprue
a. Etiology. Celiac sprue (nontropical sprue or gluten enteropathy) is caused by an allergic, immunologic, or toxic reaction to the gliadin component of gluten. There is a genetic predisposition. 

Symptoms:
– Steatorrhea, abdominal distention, flatulence, fatigue, and weight loss

Complications:
– Iron and vitamin deficiency
– Risk of lymphoma (T-cell type)

Extraintestinal manifestation:
– Dermatitis herpetiformis (a pruritic papulovesicular rash with IgA deposits at the dermoepidermal junction) 

2. Tropical sprue

Etiology. Tropical sprue is of unknown etiology, but may be  caused by enterotoxigenic E. coli. 

3. Disaccharidase deficiency is due to a deficiency of brush border enzymes. Lactase deficiency is most common. 

4. Diverticulosis Coli

- Acquired colonic diverticula are present in nearly half of the population over the age of 50
- Diverticula are associated with low-fiber, low-residue diets
- Etiology is most likely high intraluminal pressure required for propulsion of hard, small stools
- Complications include hemorrhage, acute diverticulitis, perforation, fistula formation 

Obstructive lesions

Hernias cause 15% of small intestinal obstruction. They are due to a protrusion of a serosa-lined sac through a weakness in the wall of the peritoneal cavity. They occur most commonly at the inguinal and femoral canals, at the umbilicus, and with scars. They may lead to entrapment, incarceration, and strangulation of the bowel. 

Tumors of the small bowel account for only 5% of gastrointestinal tumors. 

Benign tumors in descending order of frequency include:
leiomyomas, lipomas, adenomas (polyps), angiomas, and fibromas. Adenomatous polyps are most common in the stomach and duodenum and may be single or multiple, sessile or pedunculated. The larger the polyp, the greater the incidence of malignant transformation. 

Malignant tumors, in descending order of frequency, include: endocrine cell tumors, lymphomas, adenocarcinomas, and leiomyosarcomas. 

Idiopathic Inflammatory Bowel Disease (IBD)

- Chronic, relapsing, idiopathic inflamamtory disease of the GI tract
Crohn’s Disease
– Transmural granulomatous disease affecting any portion of the GI tract
Ulcerative Colitis
– Superficial, non-granulomatous inflammatory disease restricted to the colon

Ulcerative Colitis
- Bloody mucoid diarrhea, rarely toxic megacolon
- Can begin at any age, peaks at 20-25 years
- Annual incidence of ~10 per 100,000 in US
- Negligible risk of cancer in the first 10 years, but 1% per year risk of cancer thereafter
- Good response to total colectomy if medical therapy fails

Macroscopic
- Normal serosa
- Bowel normal thickness
- Continuous disease
- Confluent mucosal ulceration
- Pseudopolyp formation

Microscopic
- Crypt distortion + shortening
- Paneth cell metaplasia
- Diffuse mucosal inflammation
- Crypt abscesses
- Mucin depletion
- Mucosal ulceration

Crohn’s Disease

- Variable and elusive clinical presentation with diarrhea, pain, weight loss, anorexia, fever
- Can begin at any age, peaks at 15-25 years
- Annual incidence of ~3 per 100,000 in US
- Many GI complications and extracolonic manifestations
- Risk of cancer less than in UC
- Poor response to surgery 

Macroscopic
Fat wrapping
Thickened bowel wall
Skip Lesions
Stricture formation
Cobblestoned mucosa
Ulceration

Microscopic
- Cryptitis and crypt abscesses
- Transmural inflammation
- Lymphoid aggregates +/- granulomas
- “Crohn’s rosary”
- Fissuring
- Neuromuscular hyperplasia

ANAEMIA
Definition. Reduction of the hemoglobin level below the normal for the age and sex of the patient

Classification
1. Blood loss anaemia:
- Acute.
- Chronic (results in iron deficiency).

2. Deficiency anaemia:

- Iron deficiency.
- Megaloblastic anaemia-BI2 and Folic acid deficiency.
- Protein deficiency.
- Scurvy-Vitamin C deficiency.

3. Marrow dysfunction:
- Aplastic anaemia.
- Marrow infiltration.
- Liver failure.
- Renal failure.
- Collagen diseases.

4 Increased destruction (Heamolysis)
- Due to corpuscular defects.
- Due to extra corpuscular defects
 

Respiratory Pathology

A. Pulmonary infections

1. Bacterial pneumonia

a. Is an inflammatory process of infectious origin affecting the pulmonary parenchyma.

2. Bacterial infections include:

a. Streptococcus pneumoniae (most common).

b. Staphylococcus aureus.

c. Haemophilus influenzae.

d. Klebsiella pneumoniae.

e. Anaerobic bacteria from the mouth

(aspiration of oral secretions).

3. Viral infections include:

a. Influenza.

b. Parainfluenza.

c. Adenoviruses.

d. Respiratory syncytial virus.

Note: viruses can also cause pneumonia. Infection of the interstitial tissues, or interstitial pneumonia, is commonly associated with these types of infections.

Common symptoms include fever, dyspnea, and a productive cough

Two types:

(1) Lobar pneumonia

(a) Infection may spread through entire lobe(s) of lung. Intraalveolar exudates result in dense consolidations.

(b) Typical of S. pneumoniae infections.

(2) Bronchopneumonia

(a) Infection and inflammation spread through distal airways, extending from the bronchioles and alveoli. A patch distribution involving one or more lobes is observed.

(b) Typical of S. aureus, H. influenzae,and K.pneumoniae infection

Diseases that Produce a Productive Cough

Pneumonia

Lung abscess

Tuberculosis

Chronic bronchitis

Bronchiectasis

Bronchogenic carcinoma

Classification

Diseases of the respiratory system can be classified into four general areas:

• Obstructive Diseases (e.g., Emphysema, Bronchitis, Asthma)
• Restrictive Diseases (e.g., Fibrosis, Sarcoidosis, Alveolar Damage, Pleural Effusion)
• Vascular Diseases (e.g., Pulmonary Edema, Pulmonary Embolism, Pulmonary Hypertension)
• Infectious, Environmental and Other Diseases (e.g., Pneumonia, Tuberculosis, Asbestosis, Particulate Pollutants)

Varicose Veins  
- are abnormally dilated, tortuous veins produced by prolonged increase in intraluminal pressure and loss of vessel wall support. 

- The superficial veins of the leg are typically involved  

-venous pressures in these sites can be markedly elevated -> venous stasis and pedal edema (simple orthostatic edema)

-Some 10% to 20% of adult males and 25% to 33% of adult females develop lower extremity varicose  veins  

RISK FACTORS 
-> obesity  
-> Female gender  
-> pregnancy.  
-> familial tendency (premature varicosities results from imperfect venous wall development) 

 Morphology
 
- wall thinning  
- intimal fibrosis in adjacent segments 
- spotty medial calcifications (phlebosclerosis) 
- Focal intraluminal thrombosis 
- venous valve deformities (rolling and shortening) 

COMPLICATIONS
 
- stasis, congestion, edema, pain, and thrombosis 
- chronic varicose ulcers 
- embolism is very rare.