HY USMLE MSK / PHARM - #262 — Transcript

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1. 0:00it is raining here in japan and it's
2. 0:02very [ __ ] loud outside
3. 0:04so i apologize my mic is picking up the
4. 0:07sound it's either
5. 0:08make the clip or i don't have any
6. 0:11productivity so i figured i would do
7. 0:13this okay quite [ __ ] unusual
8. 0:17this question as far as leading you from
9. 0:21uh this presentation into the urinary
10. 0:23findings this is on the on the nbme
11. 0:25exams for step
12. 0:26one so this is a tangible value point
13. 0:29for you
14. 0:31this three-year-old boy has a fracture
15. 0:33of the
16. 0:34right humerus and his sclera are normal
17. 0:37when we have fractures in a kid
18. 0:38obviously it can be accidental but we
19. 0:41have to think about things such as child
20. 0:42abuse
21. 0:43osteogenesis imperfecta even
22. 0:45osteopetrosis the sclera are normal
23. 0:49implication being this is not
24. 0:50osteogenesis and perfecta there's no
25. 0:52information in the vignette as far as
26. 0:54eye contact or avoidance or cigarette
27. 0:57burns etc that could be reflective of
28. 0:59child abuse it's just not one of those
29. 1:01one of those types of questions so
30. 1:06even though this vignette is rather
31. 1:08vague to start
32. 1:10you as the student when you're doing
33. 1:12these questions
34. 1:14it's as i just said when you have a
35. 1:17pediatric case with fractures
36. 1:19your instantaneous differentials should
37. 1:21already be
38. 1:23oi child abuse osteoporosis
39. 1:27so this x-ray if you're thinking
40. 1:31osteopetrosis
41. 1:32this is showing us increased bone
42. 1:34density if you don't know what
43. 1:35osteopetrosis is
44. 1:37relax i'll explain it in a second okay
45. 1:40but this is showing us
46. 1:41increased bone density with a fracture
47. 1:43here okay
48. 1:44now even if you don't know what the
49. 1:46diagnosis is yet we keep reading
50. 1:49the deficient enzyme responsible for
51. 1:51this patient's condition you're like
52. 1:52deficient enzyme what the
53. 1:54[ __ ] like how does that relate to a
54. 1:55fracture this is osteoporosis
55. 1:58so osteoporosis also known as albers
56. 2:02schoenberg syndrome is caused by a
57. 2:05deficiency of
58. 2:06carbonic anhydrase ii which is
59. 2:10within osteoclasts of the bone as you
60. 2:13know
61. 2:13osteoclasts break down bone they do this
62. 2:17so carbonic anhydrase is an enzyme that
63. 2:20catalyzes the conversion of co2 and
64. 2:23water
65. 2:24into carbonic acid h2co3 okay
66. 2:27and there's many different isoforms
67. 2:30there is one inside of osteoclasts
68. 2:32that carbonic anhydrase ii will
69. 2:36help catalyze the production of by of
70. 2:39uh carbonic acid that will equi
71. 2:42equilibrate into a bicarb and a proton
72. 2:44the osteoclast will then concentrate
73. 2:46those protons at the interface of the
74. 2:49bone causing bone resorption
75. 2:51okay so if we have a deficiency of
76. 2:53carbonic anhydrase 2
77. 2:54we get decreased osteoclast activity
78. 2:58increased bone density despite the
79. 3:01increased bone density
80. 3:03there's weakening of the bone because
81. 3:04it's not just the density that matters
82. 3:06it's the architecture of the bone that's
83. 3:08paramount for
84. 3:10uh strength okay so
85. 3:13your initial point is when you think of
86. 3:16fractures for step one
87. 3:18step two child abuse osteogenesis
88. 3:21perfecta
89. 3:22osteopatrosis you know that carbonic
90. 3:24anhydrase is the deficient enzyme then
91. 3:26you say well
92. 3:27what kind of diuretic inhibits carbonic
93. 3:29anhydrase that's easy
94. 3:31acetazolamide okay then you say how is
95. 3:34acetazolamide going to affect these
96. 3:36urinary findings
97. 3:37not serum findings urinary findings
98. 3:41okay so this is this is important
99. 3:43because students will overread
100. 3:45or overlook that detail that we write
101. 3:48urinary
102. 3:49and the nbme does this as well i see a
103. 3:51lot of students get this wrong because
104. 3:52they're like oh [ __ ] i thought they
105. 3:54meant serum
106. 3:55not hard you just have to be careful
107. 3:56okay so
108. 3:59carbonic anhydrase is also within the
109. 4:01proximal convoluted tubular
110. 4:02cells kidney as i said we'll catalyze
111. 4:06h2o and co2 into
112. 4:08carbonic acid h2 co3 equilibrates into a
113. 4:10proton and bicarb
114. 4:11the kidney will then reabsorb that
115. 4:14bicarb
116. 4:15across the basolateral membrane and it
117. 4:17will secrete the proton into the urine
118. 4:19the urine becomes more concentra or
119. 4:21more acidic if we give our carbonic
120. 4:23anhydrase inhibitor acetazolamide
121. 4:25we're preventing the formation of
122. 4:27carbonic acid within the pct
123. 4:29cell and in turn we're preventing the
124. 4:31reabsorption
125. 4:32of bicarbonate so there's going to be
126. 4:34more bicarbonate
127. 4:36in the urine okay so it's a diuretic
128. 4:39now classically used for altitude
129. 4:41sickness because if we prevent the
130. 4:43reabsorption of
131. 4:44bicarbonate wouldn't we be making the
132. 4:48serum more acidic we'd be inducing a
133. 4:50metabolic acidosis
134. 4:52so when you go to high altitude and you
135. 4:54have increased respiratory rate
136. 4:56because you're at lower oxygen tension
137. 4:59at high altitude
138. 5:00you're going to be blowing off your co2
139. 5:02you get a respiratory alkalosis
140. 5:04you give acetazolamide you can
141. 5:06counteract that respiratory alkalosis
142. 5:08with
143. 5:09a metabolic acidosis okay because if you
144. 5:11don't give acetozoamide your kidneys
145. 5:13will take
146. 5:14at least a day to adjust your bicarb
147. 5:16whereas acetozolamide can do it pretty
148. 5:18much instantly
149. 5:18so the point of this question is if we
150. 5:21give acetazolamide
151. 5:23we are going to be increasing
152. 5:25bicarbonate in the urine
153. 5:27so we have an up arrow for urinary
154. 5:29bicarbonate obviously that means
155. 5:30increased ph
156. 5:31of the urine as well and because we have
157. 5:34a diuretic
158. 5:35we are increasing or we're increasing
159. 5:38uh urine urinary output
160. 5:42i was going to say a diuresis which is
161. 5:44just the medical word for uh urinary
162. 5:46output
163. 5:47but because we're inducing diuresis here
164. 5:50we're going to have a decreased urinary
165. 5:52osmolality okay that's what diuretics
166. 5:53are doing we're decreasing solute
167. 5:55concentration
168. 5:56so even though we've got reabsorption of
169. 5:59ions more distally in the kidney because
170. 6:01we have
171. 6:01more water to start with from the pct
172. 6:04secondary to acetazolamide ultimately
173. 6:08that net result is still going to be
174. 6:10decreased urinary osmolality okay
175. 6:12so we have up arrow bicarbonate up arrow
176. 6:14ph decreased
177. 6:16urinary osmolality and we'd have
178. 6:17increased urinary volume
179. 6:19okay as i said this isn't super
180. 6:21complicated this is on the nbmes for
181. 6:23step one so your short recapitulation
182. 6:26here is that
183. 6:27when you get a pediatric case when
184. 6:29you're dealing with a fracture
185. 6:30just remember child abuse osteogenesis
186. 6:32imperfecta albers
187. 6:34schoenberg syndrome okay three important
188. 6:36high yield differentials
189. 6:38and acetazolamide is going to be a
190. 6:40carbon carbonic anhydrase inhibitor
191. 6:43that will ultimately result in increased
192. 6:45urinary ph
193. 6:46with bicarbonate and a decreased
194. 6:48osmolality all right as i've said
195. 6:50not ultra complicated but we have to
196. 6:53uh share high yield points with you
197. 6:55because it's not about
198. 6:56what we think is entertaining it's about
199. 6:59what's actually going to give you some
200. 7:00tangible value to bump you up a point or
201. 7:02two on the
202. 7:03uh nbme and usmla so that's it for this
203. 7:07clip
204. 7:08if you liked this subscribe to my
205. 7:11channel
206. 7:12and i appreciate your time that's it