Cyanocobalamin cannot cross the blood-brain barrier, while mythelcobalamin can. This is why you should use the latter (not the former) for neuropathy caused by B12-deficiency.

.

.

ITP, TTP, HUS, & DIC as they present in investigations

The clinical presentation of neonatal meningitis is indistinguishable from that of neonatal sepsis without meningitis. The most commonly reported clinical signs are temperature instability, irritability or lethargy, and poor feeding or vomiting. Bulging fontanelle and nuchal rigidity are not common findings at the time of initial presentation but are found in approximately 25 and 15 percent of affected neonates, respectively.

Therefore, all neonates with clinical signs concerning for sepsis should undergo LP to evaluate for meningitis. Ideally, the LP should be performed before or soon after starting antibiotic therapy. However, in some cases, the neonate may be too unstable to safely undergo LP at the time of the initial diagnostic evaluation. In these instances, antimicrobials should be provided in doses sufficient for the treatment of meningitis, until the neonate is stable enough to tolerate the procedure. 

It's a paraneoplastic syndrome, caused by different mechanisms:

1- Tumor secretion of parathyroid-related peptide (PTHrP). This is also known as humoral hypercalcemia of malignancy, or pseudo-hyper-parathyroidism. It's usually seen in squamous cell carcinomas, renal, bladder, and breast cancer.

• Overproduction of active vitamin D by tumor cells. This is often seen with lymphomas (although only a minority of them cause hypercalcemia). Tumor cells express 1a-hydroxylase enzyme which converts vitamin D to its active form.

• Osteolytic lesions that release calcium from the bones. Seen in any tumor that metastasizes to the bones, but more frequently with breast cancer and multiple myeloma.

1a-hydroxylase is also expressed in macrophages. This explains why granulomatous diseases like sarcoidosis & TB can cause hypercalcemia.

.

Here are some weird dialysis complications:

AV access steal syndrome: painful ischemia of the hand secondary to the AV fistula or graft shunting blood away from the distal limb.

Hemodialysis-associated amyloidosis:
β2 microglobulin (B2M) is a normal protein in serum. It gets filtered in the kidneys but can't pass through the membranes of dialysis machines, therefore it accumulates and leads to amyloidosis. Most common presentation is carpal tunnel syndrome.

It's a chronic complication that appears after years of hemodialysis.

Dialysis disequilibrium syndrome:
Cerebral tissue takes longer to adjust to changes in osmolarity, so when you do hemodialysis (especially the first time) osmolarity will change too rapidly for cerebral tissue to adjust leading to cerebral edema.

Dialysis-associated aluminum toxicity:
Previously, we were using unfiltered water for dialysate fluid preparation which contained some amount of aluminum that normally gets filtered through the kidneys in healthy individuals, but accumulates in patients with renal failure leading to aluminum toxicity.

MHC & HLA

In humans, the Major HistoCompatibility (MHC) molecule is called the Human Leukocyte Antigen (HLA) system.

The MHC genes are codominantly expressed, which means that each individual expresses these genes from both the alleles on the cell surface. Furthermore, they are inherited as haplotypes, or one from each parent. This makes a person half identical to each of his or her parents with respect to the MHC complex. This also leads to a 25% chance that an individual might have a sibling who is HLA identical.

Human HLA genes code for 3 major class I alleles (HLA-A, -B, -C), and 3 major class II alleles (HLA-DR, -DQ, -DP). Polymorphisms in HLA, especially HLA-A, -B, and -DR loci, are important biological barriers to a successful transplantation, and may be part of many autoimmune diseases.

The MHC molecules are divided into 2 classes. The class I molecules are normally expressed on all nucleated cells, whereas the class II molecules are expressed only on the professional antigen-presenting cells (APCs), such as dendritic cells, activated macrophages, and B cells. The physiologic function of the MHC molecules is to present antigenic peptides to T cells. The class I molecules present antigenic peptides from within the cell (eg, self-antigens, intracellular viruses, tumor antigens) to CD8 T cells. The class II molecules present extracellular antigens such as extracellular bacteria to CD4 T cells.

Diabetes mellitus (DM) comes from the Greek roots 'diabetes' which means (pass through; a lot of urine) and the Latin word 'mellitus' meaning (honey-like, sweet), while the word insipidus in diabetes insipidus (DI) comes from Latin which basically means (tasteless).

This is because in times gone, when a patient urinated a lot, he was said to have diabetes. But to distinguish whether it was DM or DI, physicians had to taste the urine of the patient; if it was sweet, that is, mellitus, then it's DM. If it was tasteless, or insipidus, then it's DI.

It is possible for patients to be misdiagnosed with PCOS when in fact they have Cushing's syndrome, some form of congenital adrenal hyperplasia, or an androgen-secreting tumor as they all have a highly similar phenotype.

This is why the Rotterdam criteria explicity states that you can only make the diagnosis of PCOS after the exclusion of other etiologies.