on 27-Sep-2021 (Mon)

For example, suppose you are considering calling a friend on your cell phone. You decide that talking with her for 10 minutes would give you a benefit that you value at about $7. Your cell phone service costs you $40 per month plus $0.50 per minute for whatever calls you make. You usually talk for 100 minutes a month, so your total monthly bill is $90 ($0.50 per minute times 100 minutes, plus the $40 fixed fee). Under these circumstances, should you make the call? You might be tempted to reason as follows: “Because I pay $90 for 100 minutes of calling each month, the average minute on the phone costs me $0.90. So a 10- minute call costs $9. Because that $9 cost is greater than the $7 benefit, I am going to skip the call.” That conclusion is wrong, however. Although the average cost of a 10- minute call is $9, the marginal cost—the amount your bill increases if you make the extra call—is only $5. You will make the right decision only by compar- ing the marginal benefit and the marginal cost. Because the marginal benefit of $7 is greater than the marginal cost of $5, you should make the call. This is a prin- ciple that people innately understand: Cell phone users with unlimited minutes (that is, minutes that are free at the margin) are often prone to make long and frivolous calls.

Schizophrenia is a heterogeneous syndrome characterized by perturba- tions of language, perception, thinking, social activity, affect, and voli- tion.

Bipolar disorder is characterized by unpredictable swings in mood from mania (or hypomania) to depres- sion. Some patients suffer only from recurrent attacks of mania, which in its pure form is associated with increased psychomotor activity; excessive social extroversion; decreased need for sleep; impulsivity and impaired judgment; and expansive, grandiose, and sometimes irritable mood (Table 444-8). In severe mania, patients may experience delusions and paranoid thinking indistinguishable from schizophrenia.

One-half of patients with bipolar disorder present with a mixture of psychomotor agitation and activation with dysphoria, anxiety, and irritability. It may be difficult to distinguish mixed mania from agitated depression.

Monomers are small units which are the components of larger molecules

examples include monosaccharides such as glucose, amino acids and nucleotides

Polymers are molecules made from many monomers joined together.

Monomers are joined by a chemical bond in a condensation reaction

in a condensation reaction whereby a water molecule is eliminated.

Hydrolysis is the opposite of a condensation reaction

a condensation reaction and is when water is added to break a chemical bond between two molecules

Carbohydrates are molecules which consist only of carbon, hydrogen and oxygen

Carbohydrates are molecules which consist only of carbon, hydrogen and oxygen and they are long chains of sugar units called saccharides.

A single monomer is called a monosaccharide with a pair of monomers being called a disaccharide

Combining many monosaccharides results in the formation of a polysaccharide

These are all joined together with a glycosidic bond formed in a condensation reaction

in utero exposures to pesticides and nonsteroidal estrogens. The most important association is with cryptorchidism, which is seen with approximately 10% of testicular GCTs. Curiously, Klinefelter

Radical excision of the rectum, together with the mesorectum and associated lymph nodes, should be the aim in most cases. In the presence of widespread metastases, other means of pal- liation should be considered, such as endoluminal stenting or external beam radiotherapy, although there may still be a role for palliative resection.

In the great majority of cases, carcinoma of the rectum is an adenocarcinoma, derived from malignant transformation of the columnar rectal epithelium. The more the tumour cells retain normal shape and arrangement (well-differentiated), the less aggressive the behaviour. Conversely, the more cells of an undifferentiated type, the more aggressive the behaviour. Other poor prognostic features include vascular and perineu- ral invasion, the presence of an infiltrating (rather than push- ing) margin and tumour budding. In a small number of cases, the tumour is a primary mucoid carcinoma. The mucus lies within the cells, displacing the nucleus to the periphery, like the seal of a signet ring. Signet-ring carcinomas are rapidly growing, metastasise early and have a poor prognosis.

Before treatment can be planned, it is necessary to assess: ● the fitness of the patient; ● the extent of spread of the tumour

The anal canal commences at the level where the rectum passes through the pelvic diaphragm and ends at the anal verge. The muscular junction between the rectum and anal canal can be felt with the finger as a thickened ridge – the anorectal ‘bundle’ or ‘ring’.

It is formed by the joining of the puborectalis muscle (Figure 74.2), the deep external sphincter, conjoined longitudinal muscle and the highest part of the internal sphincter.

Puborectalis, part of the funnel-shaped muscular pelvic dia- phragm, maintains the angle between the anal canal and rec- tum and hence is an important component in the continence mechanism (Figure 74.2). The muscle derives its nerve supply from the sacral somatic nerves, and is functionally indistinct from the external anal sphincter.

Being a somatic voluntary muscle, the external sphinc- ter is red in colour and is innervated by the pudendal nerve.

Between the external sphincter muscle laterally and the lon- gitudinal muscle medially exists a potential space, the inter- sphincteric plane. This plane is important as it contains intersphincteric anal glands (see below) and is also a route for the spread of pus, which occurs along the extensions from the longitudinal muscle layer.

The longitudinal muscle is a direct continuation of the smooth muscle of the outer muscle coat of the rectum, augmented in its upper part by striated muscle fibres originating from the medial components of the pelvic floor.

During defaecation, its contraction widens the anal lumen, flattens the anal cushions, shortens the anal canal and everts the anal margin; subsequent relaxation allows the anal cush- ions to distend and thus contribute to an airtight seal.

Imperforate anus (strictly, it should be anal ‘agenesis’ or ‘atre- sia’) has historically been divided into two main groups – high and low – depending on the level of termination of the rec- tum in relation to the pelvic floor.