USMLE Biochemistry Videos & Books

2022-06-02 08:09:01 A 56-year-old man comes to the office due to difficulty swallowing for the past several months. He has the most trouble with solid foods and says, "They seem to get stuck in my throat if 1 don't chew a lot." The patient has no chest pain or heartburn and has lost 4.5 kg (10 lb) in the last 3 months. He has been an
avid hunter for many years and frequently cures the meat he eats with sodium nitrite. Physical examination is unremarkable. Endoscopy shows an ulcerated mass in the distal third of the esophagus, and biopsy samples are obtained from the mass and adjacent normal mucosa. Analysis of the samples shows accelerated cytosine deamination of chromosomal DNA in both normal and malignant epithelial cells. ओपन / कमेंट

2022-06-01 21:40:25 Educational objective: Xeroderma pigmentosum develops due to a defect in nucleotide excision repair. This disease is characterized by increased sensitivity to ultraviolet radiation and a high incidence of cutaneous malignancy. ओपन / कमेंट

2022-06-01 18:40:01 ओपन / कमेंट

2022-06-01 18:39:24 A 5-year-old girl is brought to the office by her mother because she is concerned that her daughter "sunburns too easily." The mother says the patient's skin becomes red and scaly with only minimal sun exposure. She first noticed the problem when her daughter was 7 months old during a trip to the beach.
The mother has since avoided exposing her child to excess sunlight, but finds it difficult now that the patient has begun kindergarten. Physical examination shows thin and hyperpigmented skin. She also has a few nevi on her hands that have been enlarging rapidly. ओपन / कमेंट

2022-06-01 17:08:41 ओपन / कमेंट

2022-06-01 17:08:41 A 6-year-old boy is brought to the office due to a persistent facial ulcer for the past 2 months. His mother reports that the patient has extreme sensitivity to sunlight and has developed freckles on his face, neck, and limbs since infancy. On physical examination, the skin in sun-exposed areas is dry and rough with
numerous freckles and erythematous macules. There is an ulcerated plaque on the left face; a biopsy reveals squamous cell carcinoma. Further testing leads to a diagnosis of xeroderma pigmentosum. ओपन / कमेंट

2022-05-31 20:18:01 Educational objective: Genomic imprinting refers to the phenomenon in which an offspring's genes are expressed in a parentspecific manner. Genomic imprinting is caused by DNA methylation, an epigenetic process in which genes can be silenced by attaching methyl groups to cytosine residues in the DNA molecule. ओपन / कमेंट

2022-05-31 08:17:01 ओपन / कमेंट

2022-05-31 08:17:01 A pharmaceutical researcher is evaluating a nuclear enzyme inhibitor for the treatment of an inherited disorder. During an experiment, he extracts and purifies nuclear enzymes from skin cells of an affected patient. One of these enzymes is found to catalyze the melhylation of cytosine residues in DNA using S-adenosyl-methionine (SAM) as the methyl donor. ओपन / कमेंट

2022-05-30 20:16:01 Educational objective: DNA synthesis can occur only in the 5'-3' direction. Okazaki fragments are short stretches of newly synthesized DNA that are separated by RNA primers. They are formed by the discontinuous synthesis of DNA on the lagging strand during replication. ओपन / कमेंट