In order for a drug to be cleared by the kidney, it must first be filtered in the glomeruli. Drugs with a high VD have more of the drug in the tissue that are not available to filtered by the kidney. Drugs with high protein binding won't be filtered either. So you want a drug with low Vd and low binding if you want it cleared via the kidneys and urine.

pKa is pH at which any drug is at its 50% ionized state.

Now we are alk urine i.e inc pH. when pH>pKa it will have two diff path for acidic drug & basic drug.

Acidic drug will inc its elimination (inc ionized form), basic drug will be more absorbed. so we need to know the drug is basic /acidic.

Now if u alk urine its elimination inc. so it have to be acidic. or u can know its a sodi salt of drug with CNS property i.e most like Phenobarbital (Weak acid)

so if pKa of drug is 6---at pH 7 we will start eliminating

but if pKa is 0 we need to raise pH of urine at 11 to start eliminating.at that point prev drug (pKa=6) would be totally out of system.

thats why A is the right ans (pKa = 6)

If you want to clear a drug, it is probably best that it not be bound to proteins (so that it gets filtered) and it has a low volume of distribution (so it isn't in the deep, hard to reach tissues).

alright lets do this. some of the comments were really well explained, however im going to try another route. lets deal with pKa since the other comments exaplained Vd well enough. kudos fellas/fellos.

first thing is you have to determine if its an acid or base. 90% of the time its going to be a weak acid, we simple dont formulate much medications as bases in general. also its sodium salt and CNS so you know it has to be phenobarbital. step (1) low pKa means acid (4-7). you must know this. (2) you must know that all acids start off neutral and then lose an ion --> becoming charged. and all bases start off charged and then lose an ion to become neutral (this is why we dont use basic drugs because this neutral charge has a high vd and deposits in tissues = bad in elderly). you want charged bc thats what is hydrophilic aka pee it out. (3) next use what i jjust said and apply it to this chart:

+2 = 99%

+1 = 90%

-1 = 10%

-2 = 1%

the first number represents the difference between pH and pKa (pKa is given by the question always), pH is what you changed when you alkalnized the urine. so for this question it would be: pH-pKa = high basic value (>7) minus pka ( weak acid 4-7).

now the second percentage represents the percentage of dissociated proton. so for an acid, they will become charged, this means that if the difference between the ph and pKa is over +1 then 90% of the acid will go from neutral to charged. if the ph is 2+ greater than pKA then 99% of the acid will have lost its proton and become charged

lets summarize this: were told its a weak acid = aka pKa between 4-7. we want to get to take the weak acid from its neutral form into a charged form to get rid of it. so we alkalanize the urine, because bases are higher pH you will get a more + number thus the weak acid will go from neutral to charged and be excreted.

I was wondering if it was to allow for partial CNS penetration since the drug is supposedly supposed to have CNS effect

for weak acids pH=pka + log (A-)/(HA) aka henderson hasselbach equation if you solve this, you'll get that pH - pka = log(A-)/(HA) and then again you will get 10 to power ph - pka = A-/HA. [Refer to link in comments] now considering that the ph of our body is 7.3 something, we can solve for pka 10 and pka 6 using this formula. That is for pka 6 and body ph 7.3, it will be 10 to the power of 1.3, would mean that A/HA ratio is a positive number, implying that A- > HA. whereas for when pka is 10, the A-/HA ration is 10 to power of -2.7, meaning that HA>A-.

And as already mentioned in the above comments, we need an iconic form, to eliminate the drug, it would be preferred if the pka is 6 as in that case, A-(ionic) would be in more concentration.

so what if this was inversed where the drug in question was a base and that urine was acidified.i know the pKa would need to be >7 but what about the pH

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